diff --git a/.github/workflows/smoke.yml b/.github/workflows/smoke.yml
index 5c7be225ab00..9c623f627e43 100644
--- a/.github/workflows/smoke.yml
+++ b/.github/workflows/smoke.yml
@@ -45,8 +45,9 @@ jobs:
         target:
           - aarch64-darwin
           - arm-linux-gnueabihf
-          - i386-linux-musl
           - i386-linux-gnu
+          - i386-linux-musl
+          - wasm32-unknown-wasi
           - x86_64-dragonfly
           - x86_64-freebsd
           - x86_64-netbsd
@@ -56,5 +57,9 @@ jobs:
       - name: Download Crystal source
         uses: actions/checkout@v2
 
+      - name: Make compiler (special case only for wasm32, until next release).
+        if: ${{ matrix.target == 'wasm32-unknown-wasi' }}
+        run: bin/ci with_build_env make
+
       - name: Run smoke test
         run: bin/ci with_build_env make smoke_test target=${{ matrix.target }}
diff --git a/spec/compiler/ffi/ffi_spec.cr b/spec/compiler/ffi/ffi_spec.cr
index 2ef0fe19d5da..4b70658e6bb8 100644
--- a/spec/compiler/ffi/ffi_spec.cr
+++ b/spec/compiler/ffi/ffi_spec.cr
@@ -1,4 +1,4 @@
-{% skip_file if !flag?(:unix) || flag?(:without_ffi) %}
+{% skip_file if !flag?(:unix) || flag?(:without_ffi) || flag?(:wasm32) %}
 
 require "../spec_helper"
 require "compiler/crystal/ffi"
diff --git a/spec/compiler/loader/unix_spec.cr b/spec/compiler/loader/unix_spec.cr
index e6b4be3d7d8b..cc2e9894b075 100644
--- a/spec/compiler/loader/unix_spec.cr
+++ b/spec/compiler/loader/unix_spec.cr
@@ -1,4 +1,4 @@
-{% skip_file unless flag?(:unix) %}
+{% skip_file if !flag?(:unix) || flag?(:wasm32) %}
 
 require "./spec_helper"
 require "../spec_helper"
diff --git a/spec/std/process_spec.cr b/spec/std/process_spec.cr
index c8e75b23103b..8fea7f6cf57a 100644
--- a/spec/std/process_spec.cr
+++ b/spec/std/process_spec.cr
@@ -1,3 +1,5 @@
+{% skip_file if flag?(:wasm32) %}
+
 require "spec"
 require "process"
 require "./spec_helper"
diff --git a/spec/std/signal_spec.cr b/spec/std/signal_spec.cr
index c98b4a706bea..e70278aedb3e 100644
--- a/spec/std/signal_spec.cr
+++ b/spec/std/signal_spec.cr
@@ -1,3 +1,5 @@
+{% skip_file if flag?(:wasm32) %}
+
 require "spec"
 require "signal"
 
diff --git a/spec/std/socket/addrinfo_spec.cr b/spec/std/socket/addrinfo_spec.cr
index f649274e3346..615058472525 100644
--- a/spec/std/socket/addrinfo_spec.cr
+++ b/spec/std/socket/addrinfo_spec.cr
@@ -74,8 +74,8 @@ describe Socket::Addrinfo, tags: "network" do
   end
 
   describe "Error" do
-    {% unless flag?(:win32) %}
-      # This method is not available on windows because windows support was introduced after deprecation.
+    {% unless flag?(:win32) || flag?(:wasm32) %}
+      # This method is not available on windows/wasm because windows/wasm support was introduced after deprecation.
       it ".new (deprecated)" do
         error = Socket::Addrinfo::Error.new(LibC::EAI_NONAME, "No address found", "foobar.com")
         error.os_error.should eq Errno.new(LibC::EAI_NONAME)
diff --git a/spec/std/socket/tcp_server_spec.cr b/spec/std/socket/tcp_server_spec.cr
index b01d0b3092ea..454294bef2c0 100644
--- a/spec/std/socket/tcp_server_spec.cr
+++ b/spec/std/socket/tcp_server_spec.cr
@@ -1,3 +1,5 @@
+{% skip_file if flag?(:wasm32) %}
+
 require "./spec_helper"
 require "../../support/win32"
 
diff --git a/spec/std/socket/tcp_socket_spec.cr b/spec/std/socket/tcp_socket_spec.cr
index 8a0e94fef57b..fc974f862445 100644
--- a/spec/std/socket/tcp_socket_spec.cr
+++ b/spec/std/socket/tcp_socket_spec.cr
@@ -1,3 +1,5 @@
+{% skip_file if flag?(:wasm32) %}
+
 require "./spec_helper"
 require "../../support/win32"
 
diff --git a/spec/std/string_spec.cr b/spec/std/string_spec.cr
index d0a1c7087389..cad95bc3c69b 100644
--- a/spec/std/string_spec.cr
+++ b/spec/std/string_spec.cr
@@ -2719,12 +2719,14 @@ describe "String" do
     string.should be(clone)
   end
 
-  it "allocates buffer of correct size when UInt8 is given to new (#3332)" do
-    String.new(255_u8) do |buffer|
-      LibGC.size(buffer).should be >= 255
-      {255, 0}
+  {% unless flag?(:wasm32) %}
+    it "allocates buffer of correct size when UInt8 is given to new (#3332)" do
+      String.new(255_u8) do |buffer|
+        LibGC.size(buffer).should be >= 255
+        {255, 0}
+      end
     end
-  end
+  {% end %}
 
   it "raises on String.new if returned bytesize is greater than capacity" do
     expect_raises ArgumentError, "Bytesize out of capacity bounds" do
diff --git a/src/compiler/crystal/codegen/link.cr b/src/compiler/crystal/codegen/link.cr
index 4caf0060c789..cc764678b990 100644
--- a/src/compiler/crystal/codegen/link.cr
+++ b/src/compiler/crystal/codegen/link.cr
@@ -100,7 +100,11 @@ module Crystal
 
   class Program
     def object_extension
-      has_flag?("windows") ? ".obj" : ".o"
+      case
+      when has_flag?("windows") then ".obj"
+      when has_flag?("wasm32")  then ".wasm"
+      else                           ".o"
+      end
     end
 
     def lib_flags
diff --git a/src/compiler/crystal/codegen/target.cr b/src/compiler/crystal/codegen/target.cr
index 4ab099e1a193..c89880deabda 100644
--- a/src/compiler/crystal/codegen/target.cr
+++ b/src/compiler/crystal/codegen/target.cr
@@ -96,12 +96,16 @@ class Crystal::Codegen::Target
     @environment.starts_with?("linux")
   end
 
+  def wasi?
+    @environment.starts_with?("wasi")
+  end
+
   def bsd?
     freebsd? || netbsd? || openbsd? || dragonfly?
   end
 
   def unix?
-    macos? || bsd? || linux?
+    macos? || bsd? || linux? || wasi?
   end
 
   def gnu?
@@ -144,6 +148,8 @@ class Crystal::Codegen::Target
       if cpu.empty? && !features.includes?("fp") && armhf?
         features += "+vfp2"
       end
+    when "wasm32"
+      LLVM.init_webassembly
     else
       raise Target::Error.new("Unsupported architecture for target triple: #{self}")
     end
diff --git a/src/compiler/crystal/command.cr b/src/compiler/crystal/command.cr
index a1075d1069e0..3aa966f5d694 100644
--- a/src/compiler/crystal/command.cr
+++ b/src/compiler/crystal/command.cr
@@ -247,7 +247,7 @@ class Crystal::Command
       begin
         elapsed = Time.measure do
           Process.run(output_filename, args: run_args, input: Process::Redirect::Inherit, output: Process::Redirect::Inherit, error: Process::Redirect::Inherit) do |process|
-            {% unless flag?(:win32) %}
+            {% unless flag?(:win32) || flag?(:wasm32) %}
               # Ignore the signal so we don't exit the running process
               # (the running process can still handle this signal)
               ::Signal::INT.ignore # do
diff --git a/src/compiler/crystal/compiler.cr b/src/compiler/crystal/compiler.cr
index 98e163131fea..94a75518f053 100644
--- a/src/compiler/crystal/compiler.cr
+++ b/src/compiler/crystal/compiler.cr
@@ -394,6 +394,9 @@ module Crystal
         end
 
         {cmd, nil}
+      elsif program.has_flag? "wasm32"
+        link_flags = @link_flags || ""
+        { %(wasm-ld "${@}" -o #{Process.quote_posix(output_filename)} #{link_flags} -lc #{program.lib_flags}), object_names }
       else
         link_flags = @link_flags || ""
         link_flags += " -rdynamic"
diff --git a/src/crystal/system.cr b/src/crystal/system.cr
index 88827a3f6552..953137edc730 100644
--- a/src/crystal/system.cr
+++ b/src/crystal/system.cr
@@ -8,7 +8,10 @@ module Crystal::System
   # def self.cpu_count
 end
 
-{% if flag?(:unix) %}
+{% if flag?(:wasi) %}
+  require "./system/wasi/hostname"
+  require "./system/wasi/cpucount"
+{% elsif flag?(:unix) %}
   require "./system/unix/hostname"
 
   {% if flag?(:bsd) %}
diff --git a/src/crystal/system/dir.cr b/src/crystal/system/dir.cr
index 33d983c8e411..eef9f330fbab 100644
--- a/src/crystal/system/dir.cr
+++ b/src/crystal/system/dir.cr
@@ -51,7 +51,9 @@ module Crystal::System::Dir
   # def self.delete(path : String) : Nil
 end
 
-{% if flag?(:unix) %}
+{% if flag?(:wasi) %}
+  require "./wasi/dir"
+{% elsif flag?(:unix) %}
   require "./unix/dir"
 {% elsif flag?(:win32) %}
   require "./win32/dir"
diff --git a/src/crystal/system/event_loop.cr b/src/crystal/system/event_loop.cr
index 0ca574ce82a1..d7dc351c6ae2 100644
--- a/src/crystal/system/event_loop.cr
+++ b/src/crystal/system/event_loop.cr
@@ -28,7 +28,9 @@ struct Crystal::Event
   # def add(time_span : Time::Span?) : Nil
 end
 
-{% if flag?(:unix) %}
+{% if flag?(:wasi) %}
+  require "./wasi/event_loop"
+{% elsif flag?(:unix) %}
   require "./unix/event_loop_libevent"
 {% elsif flag?(:win32) %}
   require "./win32/event_loop_iocp"
diff --git a/src/crystal/system/fiber.cr b/src/crystal/system/fiber.cr
index 23794756bc4b..230cf4cc010b 100644
--- a/src/crystal/system/fiber.cr
+++ b/src/crystal/system/fiber.cr
@@ -9,7 +9,9 @@ module Crystal::System::Fiber
   # def self.main_fiber_stack(stack_bottom : Void*) : Void*
 end
 
-{% if flag?(:unix) %}
+{% if flag?(:wasi) %}
+  require "./wasi/fiber"
+{% elsif flag?(:unix) %}
   require "./unix/fiber"
 {% elsif flag?(:win32) %}
   require "./win32/fiber"
diff --git a/src/crystal/system/file.cr b/src/crystal/system/file.cr
index f4d3ced9498c..1c5b3a0edb00 100644
--- a/src/crystal/system/file.cr
+++ b/src/crystal/system/file.cr
@@ -49,7 +49,9 @@ module Crystal::System::File
   # def file_descriptor_close
 end
 
-{% if flag?(:unix) %}
+{% if flag?(:wasi) %}
+  require "./wasi/file"
+{% elsif flag?(:unix) %}
   require "./unix/file"
 {% elsif flag?(:win32) %}
   require "./win32/file"
diff --git a/src/crystal/system/file_descriptor.cr b/src/crystal/system/file_descriptor.cr
index c484503d711c..6e5621e57786 100644
--- a/src/crystal/system/file_descriptor.cr
+++ b/src/crystal/system/file_descriptor.cr
@@ -1,4 +1,6 @@
-{% if flag?(:unix) %}
+{% if flag?(:wasi) %}
+  require "./wasi/file_descriptor"
+{% elsif flag?(:unix) %}
   require "./unix/file_descriptor"
 {% elsif flag?(:win32) %}
   require "./win32/file_descriptor"
diff --git a/src/crystal/system/group.cr b/src/crystal/system/group.cr
index 56ffdc1f39a8..dce631e8c1ab 100644
--- a/src/crystal/system/group.cr
+++ b/src/crystal/system/group.cr
@@ -1,4 +1,6 @@
-{% if flag?(:unix) %}
+{% if flag?(:wasi) %}
+  require "./wasi/group"
+{% elsif flag?(:unix) %}
   require "./unix/group"
 {% else %}
   {% raise "No Crystal::System::Group implementation available" %}
diff --git a/src/crystal/system/path.cr b/src/crystal/system/path.cr
index 17091ec3fc86..b421e1a46357 100644
--- a/src/crystal/system/path.cr
+++ b/src/crystal/system/path.cr
@@ -3,7 +3,9 @@ module Crystal::System::Path
   # def self.home : String
 end
 
-{% if flag?(:unix) %}
+{% if flag?(:wasi) %}
+  require "./wasi/path"
+{% elsif flag?(:unix) %}
   require "./unix/path"
 {% elsif flag?(:win32) %}
   require "./win32/path"
diff --git a/src/crystal/system/process.cr b/src/crystal/system/process.cr
index fca85f7a6c7e..b83d373382b7 100644
--- a/src/crystal/system/process.cr
+++ b/src/crystal/system/process.cr
@@ -70,7 +70,9 @@ module Crystal::System
   ORIGINAL_STDERR = IO::FileDescriptor.new(2, blocking: true)
 end
 
-{% if flag?(:unix) %}
+{% if flag?(:wasi) %}
+  require "./wasi/process"
+{% elsif flag?(:unix) %}
   require "./unix/process"
 {% elsif flag?(:win32) %}
   require "./win32/process"
diff --git a/src/crystal/system/random.cr b/src/crystal/system/random.cr
index 4581a3011f33..becb879cedbb 100644
--- a/src/crystal/system/random.cr
+++ b/src/crystal/system/random.cr
@@ -10,7 +10,9 @@ module Crystal::System::Random
   # def self.next_u
 end
 
-{% if flag?(:linux) %}
+{% if flag?(:wasi) %}
+  require "./wasi/random"
+{% elsif flag?(:linux) %}
   require "./unix/getrandom"
 {% elsif flag?(:openbsd) || flag?(:netbsd) %}
   require "./unix/arc4random"
diff --git a/src/crystal/system/socket.cr b/src/crystal/system/socket.cr
index fa7df866a22f..54c49f62adaa 100644
--- a/src/crystal/system/socket.cr
+++ b/src/crystal/system/socket.cr
@@ -76,7 +76,9 @@ module Crystal::System::Socket
   # private def system_tcp_keepalive_count=(val : Int)
 end
 
-{% if flag?(:unix) %}
+{% if flag?(:wasi) %}
+  require "./wasi/socket"
+{% elsif flag?(:unix) %}
   require "./unix/socket"
 {% elsif flag?(:win32) %}
   require "./win32/socket"
diff --git a/src/crystal/system/thread.cr b/src/crystal/system/thread.cr
index ffaf51555782..71f6286d9d36 100644
--- a/src/crystal/system/thread.cr
+++ b/src/crystal/system/thread.cr
@@ -28,7 +28,9 @@ end
 
 require "./thread_linked_list"
 
-{% if flag?(:unix) %}
+{% if flag?(:wasi) %}
+  require "./wasi/thread"
+{% elsif flag?(:unix) %}
   require "./unix/pthread"
   require "./unix/pthread_condition_variable"
 {% elsif flag?(:win32) %}
diff --git a/src/crystal/system/thread_mutex.cr b/src/crystal/system/thread_mutex.cr
index ced2563cefbc..1a5f3006d6eb 100644
--- a/src/crystal/system/thread_mutex.cr
+++ b/src/crystal/system/thread_mutex.cr
@@ -17,7 +17,9 @@ class Thread
   end
 end
 
-{% if flag?(:unix) %}
+{% if flag?(:wasi) %}
+  require "./wasi/thread_mutex"
+{% elsif flag?(:unix) %}
   require "./unix/pthread_mutex"
 {% elsif flag?(:win32) %}
   require "./win32/thread_mutex"
diff --git a/src/crystal/system/user.cr b/src/crystal/system/user.cr
index a065bb4e712d..ecee92c8dcb5 100644
--- a/src/crystal/system/user.cr
+++ b/src/crystal/system/user.cr
@@ -1,4 +1,6 @@
-{% if flag?(:unix) %}
+{% if flag?(:wasi) %}
+  require "./wasi/user"
+{% elsif flag?(:unix) %}
   require "./unix/user"
 {% else %}
   {% raise "No Crystal::System::User implementation available" %}
diff --git a/src/crystal/system/wasi/cpucount.cr b/src/crystal/system/wasi/cpucount.cr
new file mode 100644
index 000000000000..f6f068b09ae2
--- /dev/null
+++ b/src/crystal/system/wasi/cpucount.cr
@@ -0,0 +1,6 @@
+module Crystal::System
+  def self.cpu_count
+    # TODO: There isn't a good way to get the number of CPUs on WebAssembly
+    1
+  end
+end
diff --git a/src/crystal/system/wasi/dir.cr b/src/crystal/system/wasi/dir.cr
new file mode 100644
index 000000000000..7f6ee0056cdb
--- /dev/null
+++ b/src/crystal/system/wasi/dir.cr
@@ -0,0 +1,106 @@
+require "./wasi"
+
+module Crystal::System::Dir
+  private class DirHandle
+    property fd : LibWasi::Fd
+    property buf = Bytes.new(4096)
+    property pos = 4096u32
+    property end_pos = 4096u32
+    property cookie = 0u64
+
+    def initialize(@fd)
+    end
+
+    def fill_buffer(path)
+      err = LibWasi.fd_readdir(@fd, @buf, @buf.size, @cookie, pointerof(@end_pos))
+      raise ::File::Error.from_os_error("Error reading directory entries", err, file: path) unless err.success?
+      @pos = 0
+    end
+  end
+
+  def self.open(path : String) : DirHandle
+    parent_fd, relative_path = Wasi.find_path_preopen(path)
+
+    err = LibWasi.path_open(parent_fd, LibWasi::LookupFlags::SymlinkFollow, relative_path, LibWasi::OpenFlags::Directory, LibWasi::Rights::FdReaddir, LibWasi::Rights::None, LibWasi::FdFlags::None, out fd)
+    raise ::File::Error.from_os_error("Error opening directory", err, file: path) unless err.success?
+
+    DirHandle.new(fd)
+  end
+
+  def self.next_entry(dir, path) : Entry?
+    if dir.end_pos < dir.buf.size && dir.pos >= dir.end_pos
+      return nil
+    end
+
+    if dir.pos + sizeof(LibWasi::DirEnt) > dir.buf.size
+      dir.fill_buffer(path)
+    end
+
+    dirent = Pointer(LibWasi::DirEnt).new(dir.buf.to_unsafe.address + dir.pos).value
+
+    if dir.pos + sizeof(LibWasi::DirEnt) + dirent.d_namlen > dir.buf.size
+      if dir.pos == 0
+        dir.buf = Bytes.new(dir.buf.size * 2)
+      end
+      dir.fill_buffer(path)
+      return next_entry(dir, path)
+    end
+
+    name = String.new(dir.buf[dir.pos + sizeof(LibWasi::DirEnt), dirent.d_namlen])
+    dir.pos += sizeof(LibWasi::DirEnt) + dirent.d_namlen
+    dir.cookie = dirent.d_next
+
+    is_dir = case dirent.d_type
+             when .directory?                then true
+             when .unknown?, .symbolic_link? then nil
+             else                                 false
+             end
+
+    Entry.new(name, is_dir)
+  end
+
+  def self.rewind(dir) : Nil
+    dir.cookie = 0
+    dir.end_pos = dir.pos = dir.buf.size.to_u32
+  end
+
+  def self.close(dir, path) : Nil
+    err = LibWasi.fd_close(dir.fd)
+    raise ::File::Error.from_os_error("Error closing directory", err, file: path) unless err.success?
+  end
+
+  def self.current : String
+    unless dir = LibC.getcwd(nil, 0)
+      raise ::File::Error.from_errno("Error getting current directory", file: "./")
+    end
+
+    dir_str = String.new(dir)
+    LibC.free(dir.as(Void*))
+    dir_str
+  end
+
+  def self.current=(path : String)
+    if LibC.chdir(path.check_no_null_byte) != 0
+      raise ::File::Error.from_errno("Error while changing directory", file: path)
+    end
+
+    path
+  end
+
+  def self.tempdir
+    tmpdir = ENV["TMPDIR"]? || "/tmp"
+    tmpdir.rchop(::File::SEPARATOR)
+  end
+
+  def self.create(path : String, mode : Int32) : Nil
+    if LibC.mkdir(path.check_no_null_byte, mode) == -1
+      raise ::File::Error.from_errno("Unable to create directory", file: path)
+    end
+  end
+
+  def self.delete(path : String) : Nil
+    if LibC.rmdir(path.check_no_null_byte) == -1
+      raise ::File::Error.from_errno("Unable to remove directory", file: path)
+    end
+  end
+end
diff --git a/src/crystal/system/wasi/event_loop.cr b/src/crystal/system/wasi/event_loop.cr
new file mode 100644
index 000000000000..0d8eaf6f780a
--- /dev/null
+++ b/src/crystal/system/wasi/event_loop.cr
@@ -0,0 +1,41 @@
+# :nodoc:
+module Crystal::EventLoop
+  # Runs the event loop.
+  def self.run_once
+    raise NotImplementedError.new("Crystal::EventLoop.run_once")
+  end
+
+  # Create a new resume event for a fiber.
+  def self.create_resume_event(fiber : Fiber) : Crystal::Event
+    raise NotImplementedError.new("Crystal::EventLoop.create_resume_event")
+  end
+
+  # Creates a timeout_event.
+  def self.create_timeout_event(fiber) : Crystal::Event
+    raise NotImplementedError.new("Crystal::EventLoop.create_timeout_event")
+  end
+
+  # Creates a write event for a file descriptor.
+  def self.create_fd_write_event(io : IO::Evented, edge_triggered : Bool = false) : Crystal::Event
+    raise NotImplementedError.new("Crystal::EventLoop.create_fd_write_event")
+  end
+
+  # Creates a read event for a file descriptor.
+  def self.create_fd_read_event(io : IO::Evented, edge_triggered : Bool = false) : Crystal::Event
+    raise NotImplementedError.new("Crystal::EventLoop.create_fd_read_event")
+  end
+end
+
+struct Crystal::Event
+  def add(timeout : LibC::Timeval? = nil)
+  end
+
+  def add(timeout : Time::Span)
+  end
+
+  def free
+  end
+
+  def delete
+  end
+end
diff --git a/src/crystal/system/wasi/fiber.cr b/src/crystal/system/wasi/fiber.cr
new file mode 100644
index 000000000000..cabb06a9cd6e
--- /dev/null
+++ b/src/crystal/system/wasi/fiber.cr
@@ -0,0 +1,9 @@
+module Crystal::System::Fiber
+  def self.allocate_stack(stack_size) : Void*
+    LibC.malloc(stack_size)
+  end
+
+  def self.free_stack(stack : Void*, stack_size) : Nil
+    LibC.free(stack)
+  end
+end
diff --git a/src/crystal/system/wasi/file.cr b/src/crystal/system/wasi/file.cr
new file mode 100644
index 000000000000..c3c77dae2714
--- /dev/null
+++ b/src/crystal/system/wasi/file.cr
@@ -0,0 +1,40 @@
+require "../unix/file"
+
+# :nodoc:
+module Crystal::System::File
+  def self.chmod(path, mode)
+    raise NotImplementedError.new "Crystal::System::File.chmod"
+  end
+
+  def self.chown(path, uid : Int, gid : Int, follow_symlinks)
+    raise NotImplementedError.new "Crystal::System::File.chown"
+  end
+
+  def self.real_path(path)
+    raise NotImplementedError.new "Crystal::System::File.real_path"
+  end
+
+  def self.utime(atime : ::Time, mtime : ::Time, filename : String) : Nil
+    raise NotImplementedError.new "Crystal::System::File.utime"
+  end
+
+  private def system_flock_shared(blocking)
+    raise NotImplementedError.new "Crystal::System::File#system_flock_shared"
+  end
+
+  private def system_flock_exclusive(blocking)
+    raise NotImplementedError.new "Crystal::System::File#system_flock_exclusive"
+  end
+
+  private def system_flock_unlock
+    raise NotImplementedError.new "Crystal::System::File#system_flock_unlock"
+  end
+
+  private def flock(op : LibC::FlockOp, blocking : Bool = true)
+    raise NotImplementedError.new "Crystal::System::File#flock"
+  end
+
+  def self.mktemp(prefix, suffix, dir) : {LibC::Int, String}
+    raise NotImplementedError.new "Crystal::System::File.mktemp"
+  end
+end
diff --git a/src/crystal/system/wasi/file_descriptor.cr b/src/crystal/system/wasi/file_descriptor.cr
new file mode 100644
index 000000000000..7a375e89a136
--- /dev/null
+++ b/src/crystal/system/wasi/file_descriptor.cr
@@ -0,0 +1,17 @@
+require "../unix/file_descriptor"
+
+# :nodoc:
+module Crystal::System::FileDescriptor
+  def self.from_stdio(fd)
+    # TODO: WASI doesn't offer a way to detect if a 'fd' is a TTY.
+    IO::FileDescriptor.new(fd).tap(&.flush_on_newline=(true))
+  end
+
+  def self.pipe(read_blocking, write_blocking)
+    raise NotImplementedError.new "Crystal::System::FileDescriptor.pipe"
+  end
+
+  private def system_reopen(other : IO::FileDescriptor)
+    raise NotImplementedError.new "Crystal::System::FileDescriptor.system_reopen"
+  end
+end
diff --git a/src/crystal/system/wasi/group.cr b/src/crystal/system/wasi/group.cr
new file mode 100644
index 000000000000..0aa09bd40aa8
--- /dev/null
+++ b/src/crystal/system/wasi/group.cr
@@ -0,0 +1,9 @@
+module Crystal::System::Group
+  private def from_name?(groupname : String)
+    raise NotImplementedError.new("Crystal::System::Group#from_name?")
+  end
+
+  private def from_id?(groupid : String)
+    raise NotImplementedError.new("Crystal::System::Group#from_id?")
+  end
+end
diff --git a/src/crystal/system/wasi/hostname.cr b/src/crystal/system/wasi/hostname.cr
new file mode 100644
index 000000000000..e59611d6a0a4
--- /dev/null
+++ b/src/crystal/system/wasi/hostname.cr
@@ -0,0 +1,5 @@
+module Crystal::System
+  def self.hostname
+    raise NotImplementedError.new("Crystal::System.hostname")
+  end
+end
diff --git a/src/crystal/system/wasi/lib_wasi.cr b/src/crystal/system/wasi/lib_wasi.cr
new file mode 100644
index 000000000000..a958b6e46206
--- /dev/null
+++ b/src/crystal/system/wasi/lib_wasi.cr
@@ -0,0 +1,102 @@
+lib LibWasi
+  alias Fd = Int32
+  alias Size = UInt32
+  alias FileSize = UInt64
+
+  struct PrestatDir
+    pr_name_len : Size
+  end
+
+  union PrestatUnion
+    dir : PrestatDir
+  end
+
+  enum PrestatTag : UInt8
+    Dir
+  end
+
+  struct Prestat
+    tag : PrestatTag
+    value : PrestatUnion
+  end
+
+  @[Flags]
+  enum LookupFlags : UInt32
+    SymlinkFollow
+  end
+
+  @[Flags]
+  enum OpenFlags : UInt16
+    Creat
+    Directory
+    Excl
+    Trunc
+  end
+
+  @[Flags]
+  enum FdFlags : UInt16
+    Append   # Append mode: Data written to the file is always appended to the file's end.
+    Dsync    # Write according to synchronized I/O data integrity completion. Only the data stored in the file is synchronized.
+    NonBlock # Non-blocking mode.
+    RSync    # Synchronized read I/O operations.
+    Sync     # Write according to synchronized I/O file integrity completion. In addition to synchronizing the data stored in the file, the implementation may also synchronously update the file's metadata.
+  end
+
+  @[Flags]
+  enum Rights : UInt64
+    FdDatasync           # The right to invoke fd_datasync. If path_open is set, includes the right to invoke path_open with fdflags::dsync.
+    FdRead               # The right to invoke fd_read and sock_recv. If rights::fd_seek is set, includes the right to invoke fd_pread.
+    FdSeek               # The right to invoke fd_seek. This flag implies rights::fd_tell.
+    FdFdstatSetFlags     # The right to invoke fd_fdstat_set_flags.
+    FdSync               # The right to invoke fd_sync. If path_open is set, includes the right to invoke path_open with fdflags::rsync and fdflags::dsync.
+    FdTell               # The right to invoke fd_seek in such a way that the file offset remains unaltered (i.e., whence::cur with offset zero), or to invoke fd_tell.
+    FdWrite              # The right to invoke fd_write and sock_send. If rights::fd_seek is set, includes the right to invoke fd_pwrite.
+    FdAdvise             # The right to invoke fd_advise.
+    FdAllocate           # The right to invoke fd_allocate.
+    PathCreateDirectory  # The right to invoke path_create_directory.
+    PathCreateFile       # If path_open is set, the right to invoke path_open with oflags::creat.
+    PathLinkSource       # The right to invoke path_link with the file descriptor as the source directory.
+    PathLinkTarget       # The right to invoke path_link with the file descriptor as the target directory.
+    PathOpen             # The right to invoke path_open.
+    FdReaddir            # The right to invoke fd_readdir.
+    PathReadlink         # The right to invoke path_readlink.
+    PathRenameSource     # The right to invoke path_rename with the file descriptor as the source directory.
+    PathRenameTarget     # The right to invoke path_rename with the file descriptor as the target directory.
+    PathFilestatGet      # The right to invoke path_filestat_get.
+    PathFilestatSetSize  # The right to change a file's size (there is no path_filestat_set_size). If path_open is set, includes the right to invoke path_open with oflags::trunc.
+    PathFilestatSetTimes # The right to invoke path_filestat_set_times.
+    FdFilestatGet        # The right to invoke fd_filestat_get.
+    FdFilestatSetSize    # The right to invoke fd_filestat_set_size.
+    FdFilestatSetTimes   # The right to invoke fd_filestat_set_times.
+    PathSymlink          # The right to invoke path_symlink.
+    PathRemoveDirectory  # The right to invoke path_remove_directory.
+    PathUnlinkFile       # The right to invoke path_unlink_file.
+    PollFdReadwrite      # If rights::fd_read is set, includes the right to invoke poll_oneoff to subscribe to eventtype::fd_read. If rights::fd_write is set, includes the right to invoke poll_oneoff to subscribe to eventtype::fd_write.
+    SockShutdown         # The right to invoke sock_shutdown.
+  end
+
+  enum FileType : UInt8
+    Unknown         # The type of the file descriptor or file is unknown or is different from any of the other types specified.
+    BlockDevice     # The file descriptor or file refers to a block device inode.
+    CharacterDevice # The file descriptor or file refers to a character device inode.
+    Directory       # The file descriptor or file refers to a directory inode.
+    RegularFile     # The file descriptor or file refers to a regular file inode.
+    SocketDgram     # The file descriptor or file refers to a datagram socket.
+    SocketStream    # The file descriptor or file refers to a byte-stream socket.
+    SymbolicLink    # The file refers to a symbolic link inode.
+  end
+
+  struct DirEnt
+    d_next : UInt64
+    d_ino : UInt64
+    d_namlen : UInt32
+    d_type : FileType
+  end
+
+  fun fd_prestat_get = __wasi_fd_prestat_get(fd : Fd, stat : Prestat*) : WasiError
+  fun fd_prestat_dir_name = __wasi_fd_prestat_dir_name(fd : Fd, path : UInt8*, len : Size) : WasiError
+  fun path_open = __wasi_path_open(fd : Fd, dirflags : LookupFlags, path : UInt8*, oflags : OpenFlags, fs_rights_base : Rights, fs_rights_inheriting : Rights, fdflags : FdFlags, ret : Fd*) : WasiError
+  fun fd_readdir = __wasi_fd_readdir(fd : Fd, buf : UInt8*, len : Size, cookie : UInt64, ret : Size*) : WasiError
+  fun fd_close = __wasi_fd_close(fd : Fd) : WasiError
+  fun random_get = __wasi_random_get(buf : UInt8*, len : Size) : WasiError
+end
diff --git a/src/crystal/system/wasi/path.cr b/src/crystal/system/wasi/path.cr
new file mode 100644
index 000000000000..5a570aa71dc8
--- /dev/null
+++ b/src/crystal/system/wasi/path.cr
@@ -0,0 +1,5 @@
+module Crystal::System::Path
+  def self.home : String
+    ENV["HOME"]
+  end
+end
diff --git a/src/crystal/system/wasi/process.cr b/src/crystal/system/wasi/process.cr
new file mode 100644
index 000000000000..a9d5d0e9fa69
--- /dev/null
+++ b/src/crystal/system/wasi/process.cr
@@ -0,0 +1,78 @@
+require "c/stdlib"
+require "c/unistd"
+
+struct Crystal::System::Process
+  getter pid : LibC::PidT
+
+  def initialize(@pid : LibC::PidT)
+  end
+
+  def release
+    raise NotImplementedError.new("Process#release")
+  end
+
+  def wait
+    raise NotImplementedError.new("Process#wait")
+  end
+
+  def exists?
+    raise NotImplementedError.new("Process#exists?")
+  end
+
+  def terminate
+    raise NotImplementedError.new("Process#terminate")
+  end
+
+  def self.exit(status)
+    LibC.exit(status)
+  end
+
+  def self.pid
+    # TODO: WebAssembly doesn't have the concept of processes.
+    1
+  end
+
+  def self.pgid
+    raise NotImplementedError.new("Process.pgid")
+  end
+
+  def self.pgid(pid)
+    raise NotImplementedError.new("Process.pgid")
+  end
+
+  def self.ppid
+    raise NotImplementedError.new("Process.ppid")
+  end
+
+  def self.signal(pid, signal)
+    raise NotImplementedError.new("Process.signal")
+  end
+
+  def self.exists?(pid)
+    raise NotImplementedError.new("Process.exists?")
+  end
+
+  def self.times
+    raise NotImplementedError.new("Process.times")
+  end
+
+  def self.fork(*, will_exec = false)
+    raise NotImplementedError.new("Process.fork")
+  end
+
+  def self.spawn(command_args, env, clear_env, input, output, error, chdir)
+    raise NotImplementedError.new("Process.spawn")
+  end
+
+  def self.prepare_args(command : String, args : Enumerable(String)?, shell : Bool) : Array(String)
+    raise NotImplementedError.new("Process.prepare_args")
+  end
+
+  def self.replace(command_args, env, clear_env, input, output, error, chdir)
+    raise NotImplementedError.new("Process.replace")
+  end
+
+  def self.chroot(path)
+    raise NotImplementedError.new("Process.chroot")
+  end
+end
diff --git a/src/crystal/system/wasi/random.cr b/src/crystal/system/wasi/random.cr
new file mode 100644
index 000000000000..72649fac3e7c
--- /dev/null
+++ b/src/crystal/system/wasi/random.cr
@@ -0,0 +1,14 @@
+require "./lib_wasi"
+
+module Crystal::System::Random
+  def self.random_bytes(buf : Bytes) : Nil
+    err = LibWasi.random_get(buf, buf.size)
+    raise RuntimeError.from_os_error("random_get", err) unless err.success?
+  end
+
+  def self.next_u : UInt8
+    buf = uninitialized UInt8[1]
+    random_bytes(buf.to_slice)
+    buf.unsafe_as(UInt8)
+  end
+end
diff --git a/src/crystal/system/wasi/socket.cr b/src/crystal/system/wasi/socket.cr
new file mode 100644
index 000000000000..974613b847c4
--- /dev/null
+++ b/src/crystal/system/wasi/socket.cr
@@ -0,0 +1,185 @@
+require "c/netdb"
+require "c/netinet/tcp"
+require "c/sys/socket"
+require "io/evented"
+
+module Crystal::System::Socket
+  include IO::Evented
+
+  alias Handle = Int32
+
+  private def create_handle(family, type, protocol, blocking) : Handle
+    raise NotImplementedError.new "Crystal::System::Socket#create_handle"
+  end
+
+  private def initialize_handle(fd)
+  end
+
+  private def system_connect(addr, timeout = nil, &)
+    raise NotImplementedError.new "Crystal::System::Socket#system_connect"
+  end
+
+  # Tries to bind the socket to a local address.
+  # Yields an `Socket::BindError` if the binding failed.
+  private def system_bind(addr, addrstr, &)
+    raise NotImplementedError.new "Crystal::System::Socket#system_bind"
+  end
+
+  private def system_listen(backlog, &)
+    raise NotImplementedError.new "Crystal::System::Socket#system_listen"
+  end
+
+  private def system_accept
+    (raise NotImplementedError.new "Crystal::System::Socket#system_accept").as(Int32)
+  end
+
+  private def system_send(bytes : Bytes) : Int32
+    evented_send(bytes, "Error sending datagram") do |slice|
+      LibC.send(fd, slice.to_unsafe.as(Void*), slice.size, 0)
+    end
+  end
+
+  private def system_send_to(bytes : Bytes, addr : ::Socket::Address)
+    raise NotImplementedError.new "Crystal::System::Socket#system_send_to"
+  end
+
+  private def system_receive(bytes)
+    raise NotImplementedError.new "Crystal::System::Socket#system_receive"
+  end
+
+  private def system_close_read
+    if LibC.shutdown(fd, LibC::SHUT_RD) != 0
+      raise ::Socket::Error.from_errno("shutdown read")
+    end
+  end
+
+  private def system_close_write
+    if LibC.shutdown(fd, LibC::SHUT_WR) != 0
+      raise ::Socket::Error.from_errno("shutdown write")
+    end
+  end
+
+  private def system_reuse_port?
+    raise NotImplementedError.new "Crystal::System::Socket#system_reuse_port?"
+  end
+
+  private def system_reuse_port=(val : Bool)
+    raise NotImplementedError.new "Crystal::System::Socket#system_reuse_port="
+  end
+
+  private def system_linger
+    raise NotImplementedError.new "Crystal::System::Socket#system_linger"
+  end
+
+  private def system_linger=(val)
+    raise NotImplementedError.new "Crystal::System::Socket#system_linge="
+  end
+
+  private def system_getsockopt(fd, optname, optval, level = LibC::SOL_SOCKET, &)
+    raise NotImplementedError.new "Crystal::System::Socket#system_getsockopt"
+  end
+
+  private def system_setsockopt(fd, optname, optval, level = LibC::SOL_SOCKET)
+    raise NotImplementedError.new "Crystal::System::Socket#system_setsockopt"
+  end
+
+  private def system_blocking?
+    fcntl(LibC::F_GETFL) & LibC::O_NONBLOCK == 0
+  end
+
+  private def system_blocking=(value)
+    flags = fcntl(LibC::F_GETFL)
+    if value
+      flags &= ~LibC::O_NONBLOCK
+    else
+      flags |= LibC::O_NONBLOCK
+    end
+    fcntl(LibC::F_SETFL, flags)
+  end
+
+  private def system_close_on_exec?
+    flags = fcntl(LibC::F_GETFD)
+    (flags & LibC::FD_CLOEXEC) == LibC::FD_CLOEXEC
+  end
+
+  private def system_close_on_exec=(arg : Bool)
+    fcntl(LibC::F_SETFD, arg ? LibC::FD_CLOEXEC : 0)
+    arg
+  end
+
+  def self.fcntl(fd, cmd, arg = 0)
+    r = LibC.fcntl fd, cmd, arg
+    raise ::Socket::Error.from_errno("fcntl() failed") if r == -1
+    r
+  end
+
+  private def system_tty?
+    LibC.isatty(fd) == 1
+  end
+
+  private def unbuffered_read(slice : Bytes)
+    evented_read(slice, "Error reading socket") do
+      LibC.recv(fd, slice, slice.size, 0).to_i32
+    end
+  end
+
+  private def unbuffered_write(slice : Bytes)
+    evented_write(slice, "Error writing to socket") do |slice|
+      LibC.send(fd, slice, slice.size, 0)
+    end
+  end
+
+  private def system_close
+    # Perform libevent cleanup before LibC.close.
+    # Using a file descriptor after it has been closed is never defined and can
+    # always lead to undefined results. This is not specific to libevent.
+    evented_close
+
+    # Clear the @volatile_fd before actually closing it in order to
+    # reduce the chance of reading an outdated fd value
+    fd = @volatile_fd.swap(-1)
+
+    ret = LibC.close(fd)
+
+    if ret != 0
+      case Errno.value
+      when Errno::EINTR, Errno::EINPROGRESS
+        # ignore
+      else
+        raise ::Socket::Error.from_errno("Error closing socket")
+      end
+    end
+  end
+
+  private def system_local_address
+    raise NotImplementedError.new "Crystal::System::Socket#system_local_address"
+  end
+
+  private def system_remote_address
+    raise NotImplementedError.new "Crystal::System::Socket#system_remote_address"
+  end
+
+  private def system_tcp_keepalive_idle
+    raise NotImplementedError.new("Crystal::System::Socket#system_tcp_keepalive_idle")
+  end
+
+  private def system_tcp_keepalive_idle=(val : Int)
+    raise NotImplementedError.new("Crystal::System::Socket#system_tcp_keepalive_idle=")
+  end
+
+  private def system_tcp_keepalive_interval
+    raise NotImplementedError.new("Crystal::System::Socket#system_tcp_keepalive_interval")
+  end
+
+  private def system_tcp_keepalive_interval=(val : Int)
+    raise NotImplementedError.new("Crystal::System::Socket#system_tcp_keepalive_interval=")
+  end
+
+  private def system_tcp_keepalive_count
+    raise NotImplementedError.new("Crystal::System::Socket#system_tcp_keepalive_count")
+  end
+
+  private def system_tcp_keepalive_count=(val : Int)
+    raise NotImplementedError.new("Crystal::System::Socket#system_tcp_keepalive_count=")
+  end
+end
diff --git a/src/crystal/system/wasi/thread.cr b/src/crystal/system/wasi/thread.cr
new file mode 100644
index 000000000000..b10439852f55
--- /dev/null
+++ b/src/crystal/system/wasi/thread.cr
@@ -0,0 +1,73 @@
+class Thread
+  @main_fiber : Fiber?
+
+  def initialize
+    @main_fiber = Fiber.new(stack_address, self)
+
+    # TODO: Create thread
+  end
+
+  def initialize(&func : ->)
+    initialize
+  end
+
+  def join : Nil
+    raise NotImplementedError.new("Thread#join")
+  end
+
+  def self.yield : Nil
+    raise NotImplementedError.new("Thread.yield")
+  end
+
+  @@current = Thread.new
+
+  # Associates the Thread object to the running system thread.
+  protected def self.current=(@@current : Thread) : Thread
+  end
+
+  # Returns the Thread object associated to the running system thread.
+  def self.current : Thread
+    @@current
+  end
+
+  # Create the thread object for the current thread (aka the main thread of the
+  # process).
+  #
+  # TODO: consider moving to `kernel.cr` or `crystal/main.cr`
+  self.current = new
+
+  # Returns the Fiber representing the thread's main stack.
+  def main_fiber
+    @main_fiber.not_nil!
+  end
+
+  # :nodoc:
+  def scheduler
+    @scheduler ||= Crystal::Scheduler.new(main_fiber)
+  end
+
+  protected def start
+    raise NotImplementedError.new("Thread#start")
+  end
+
+  private def stack_address : Void*
+    # TODO: Implement
+    Pointer(Void).null
+  end
+
+  # :nodoc:
+  # TODO: Implement
+  class ConditionVariable
+    def signal : Nil
+    end
+
+    def broadcast : Nil
+    end
+
+    def wait(mutex : Thread::Mutex) : Nil
+    end
+
+    def wait(mutex : Thread::Mutex, time : Time::Span, &)
+    end
+  end
+end
diff --git a/src/crystal/system/wasi/thread_mutex.cr b/src/crystal/system/wasi/thread_mutex.cr
new file mode 100644
index 000000000000..0d90bebb67f8
--- /dev/null
+++ b/src/crystal/system/wasi/thread_mutex.cr
@@ -0,0 +1,17 @@
+# TODO: Implement
+class Thread
+  class Mutex
+    def lock
+    end
+
+    def try_lock
+    end
+
+    def unlock
+    end
+
+    def synchronize
+      yield
+    end
+  end
+end
diff --git a/src/crystal/system/wasi/user.cr b/src/crystal/system/wasi/user.cr
new file mode 100644
index 000000000000..06415897000e
--- /dev/null
+++ b/src/crystal/system/wasi/user.cr
@@ -0,0 +1,9 @@
+module Crystal::System::User
+  private def from_username?(username : String)
+    raise NotImplementedError.new("Crystal::System::User#from_username?")
+  end
+
+  private def from_id?(id : String)
+    raise NotImplementedError.new("Crystal::System::User#from_id?")
+  end
+end
diff --git a/src/crystal/system/wasi/wasi.cr b/src/crystal/system/wasi/wasi.cr
new file mode 100644
index 000000000000..3db4f600df1c
--- /dev/null
+++ b/src/crystal/system/wasi/wasi.cr
@@ -0,0 +1,52 @@
+require "./lib_wasi"
+
+module Crystal::System::Wasi
+  PREOPENS = begin
+    preopens = [] of {String, String, LibWasi::Fd}
+
+    # Skip stdin, stdout, and stderr, and count up until we reach an invalid file descriptor.
+    (3..).each do |fd|
+      stat = uninitialized LibWasi::Prestat
+      err = LibWasi.fd_prestat_get(fd, pointerof(stat))
+
+      break unless err.success?
+
+      next unless stat.tag.dir?
+
+      len = stat.value.dir.pr_name_len
+
+      name = String.new(len + 1) do |buffer|
+        err = LibWasi.fd_prestat_dir_name(fd, buffer, len)
+        raise RuntimeError.from_os_error("fd_prestat_dir_name", err) unless err.success?
+        buffer[len] = 0
+        len = LibC.strlen(buffer)
+        {len, 0}
+      end
+
+      path = ::Path[name].expand.to_s
+      preopens << {path, path.ends_with?("/") ? path : path + "/", fd}
+    end
+
+    # Preopens added later take priority over preopens added earlier.
+    preopens.reverse!
+    # Preopens of longer prefix take priority over shorter prefixes.
+    preopens.sort_by! { |entry| -entry[0].size }
+
+    preopens
+  end
+
+  def self.find_path_preopen(path)
+    path = ::Path[path].expand.to_s
+    PREOPENS.each do |preopen|
+      case path
+      when preopen[0]
+        return {preopen[2], "."}
+      when .starts_with? preopen[1]
+        return {preopen[2], path[preopen[1].size..-1]}
+      end
+    end
+
+    # If we can't find a preopen for it, indicate that we lack capabilities.
+    raise RuntimeError.from_os_error(nil, WasiError::NOTCAPABLE)
+  end
+end
diff --git a/src/errno.cr b/src/errno.cr
index 9a3afe5b5899..cc05673b12f9 100644
--- a/src/errno.cr
+++ b/src/errno.cr
@@ -4,6 +4,8 @@ require "c/string"
 lib LibC
   {% if flag?(:linux) || flag?(:dragonfly) %}
     fun __errno_location : Int*
+  {% elsif flag?(:wasi) %}
+    $errno : Int
   {% elsif flag?(:darwin) || flag?(:freebsd) %}
     fun __error : Int*
   {% elsif flag?(:netbsd) || flag?(:openbsd) %}
@@ -43,6 +45,8 @@ enum Errno
   def self.value : self
     {% if flag?(:linux) || flag?(:dragonfly) %}
       Errno.new LibC.__errno_location.value
+    {% elsif flag?(:wasi) %}
+      Errno.new LibC.errno
     {% elsif flag?(:darwin) || flag?(:bsd) %}
       Errno.new LibC.__error.value
     {% elsif flag?(:win32) %}
diff --git a/src/exception/call_stack.cr b/src/exception/call_stack.cr
index b59ea603235f..b1e57010d0d3 100644
--- a/src/exception/call_stack.cr
+++ b/src/exception/call_stack.cr
@@ -2,6 +2,8 @@
   require "./call_stack/stackwalk"
 {% elsif flag?(:interpreted) %}
   require "./call_stack/interpreter"
+{% elsif flag?(:wasm32) %}
+  require "./call_stack/null"
 {% else %}
   require "./call_stack/libunwind"
 {% end %}
@@ -38,62 +40,66 @@ struct Exception::CallStack
   end
 
   private def decode_backtrace
-    show_full_info = ENV["CRYSTAL_CALLSTACK_FULL_INFO"]? == "1"
+    {% if flag?(:wasm32) %}
+      [] of String
+    {% else %}
+      show_full_info = ENV["CRYSTAL_CALLSTACK_FULL_INFO"]? == "1"
 
-    @callstack.compact_map do |ip|
-      pc = CallStack.decode_address(ip)
+      @callstack.compact_map do |ip|
+        pc = CallStack.decode_address(ip)
 
-      file, line_number, column_number = CallStack.decode_line_number(pc)
+        file, line_number, column_number = CallStack.decode_line_number(pc)
 
-      if file && file != "??"
-        next if @@skip.includes?(file)
+        if file && file != "??"
+          next if @@skip.includes?(file)
 
-        # Turn to relative to the current dir, if possible
-        if current_dir = CURRENT_DIR
-          if rel = Path[file].relative_to?(current_dir)
-            rel = rel.to_s
-            file = rel unless rel.starts_with?("..")
+          # Turn to relative to the current dir, if possible
+          if current_dir = CURRENT_DIR
+            if rel = Path[file].relative_to?(current_dir)
+              rel = rel.to_s
+              file = rel unless rel.starts_with?("..")
+            end
           end
-        end
 
-        file_line_column = file
-        unless line_number == 0
-          file_line_column = "#{file_line_column}:#{line_number}"
-          file_line_column = "#{file_line_column}:#{column_number}" unless column_number == 0
+          file_line_column = file
+          unless line_number == 0
+            file_line_column = "#{file_line_column}:#{line_number}"
+            file_line_column = "#{file_line_column}:#{column_number}" unless column_number == 0
+          end
         end
-      end
 
-      if name = CallStack.decode_function_name(pc)
-        function = name
-      elsif frame = CallStack.decode_frame(ip)
-        _, function, file = frame
-        # Crystal methods (their mangled name) start with `*`, so
-        # we remove that to have less clutter in the output.
-        function = function.lchop('*')
-      else
-        function = "??"
-      end
-
-      if file_line_column
-        if show_full_info && (frame = CallStack.decode_frame(ip))
-          _, sname, _ = frame
-          line = "#{file_line_column} in '#{sname}'"
+        if name = CallStack.decode_function_name(pc)
+          function = name
+        elsif frame = CallStack.decode_frame(ip)
+          _, function, file = frame
+          # Crystal methods (their mangled name) start with `*`, so
+          # we remove that to have less clutter in the output.
+          function = function.lchop('*')
         else
-          line = "#{file_line_column} in '#{function}'"
+          function = "??"
         end
-      else
-        if file == "??" && function == "??"
-          line = "???"
+
+        if file_line_column
+          if show_full_info && (frame = CallStack.decode_frame(ip))
+            _, sname, _ = frame
+            line = "#{file_line_column} in '#{sname}'"
+          else
+            line = "#{file_line_column} in '#{function}'"
+          end
         else
-          line = "#{file} in '#{function}'"
+          if file == "??" && function == "??"
+            line = "???"
+          else
+            line = "#{file} in '#{function}'"
+          end
         end
-      end
 
-      if show_full_info
-        line = "#{line} at 0x#{ip.address.to_s(16)}"
-      end
+        if show_full_info
+          line = "#{line} at 0x#{ip.address.to_s(16)}"
+        end
 
-      line
-    end
+        line
+      end
+    {% end %}
   end
 end
diff --git a/src/exception/call_stack/elf.cr b/src/exception/call_stack/elf.cr
index 8c7bb45b3219..cdedae66dd42 100644
--- a/src/exception/call_stack/elf.cr
+++ b/src/exception/call_stack/elf.cr
@@ -1,5 +1,7 @@
 require "crystal/elf"
-require "c/link"
+{% unless flag?(:wasm32) %}
+  require "c/link"
+{% end %}
 
 struct Exception::CallStack
   protected def self.load_debug_info_impl
diff --git a/src/exception/call_stack/null.cr b/src/exception/call_stack/null.cr
index 6b9d2ab6820f..e00cec4943e8 100644
--- a/src/exception/call_stack/null.cr
+++ b/src/exception/call_stack/null.cr
@@ -10,4 +10,12 @@ struct Exception::CallStack
   def self.decode_function_name(pc)
     nil
   end
+
+  protected def self.decode_frame(pc)
+    nil
+  end
+
+  protected def self.unwind : Array(Void*)
+    [] of Void*
+  end
 end
diff --git a/src/fiber/context/wasm32.cr b/src/fiber/context/wasm32.cr
new file mode 100644
index 000000000000..0d8372d3e325
--- /dev/null
+++ b/src/fiber/context/wasm32.cr
@@ -0,0 +1,15 @@
+{% skip_file unless flag?(:wasm32) %}
+
+class Fiber
+  # :nodoc:
+  def makecontext(stack_ptr, fiber_main)
+    # TODO: Implement this using Binaryen Asyncify
+  end
+
+  # :nodoc:
+  @[NoInline]
+  @[Naked]
+  def self.swapcontext(current_context, new_context) : Nil
+    # TODO: Implement this using Binaryen Asyncify
+  end
+end
diff --git a/src/gc.cr b/src/gc.cr
index 238c99c1b574..4bf29345a6b6 100644
--- a/src/gc.cr
+++ b/src/gc.cr
@@ -98,7 +98,7 @@ module GC
   end
 end
 
-{% if flag?(:gc_none) %}
+{% if flag?(:gc_none) || flag?(:wasm32) %}
   require "gc/none"
 {% else %}
   require "gc/boehm"
diff --git a/src/gc/boehm.cr b/src/gc/boehm.cr
index a5ed138d4151..be143e1f4afb 100644
--- a/src/gc/boehm.cr
+++ b/src/gc/boehm.cr
@@ -93,7 +93,7 @@ lib LibGC
 
   fun size = GC_size(addr : Void*) : LibC::SizeT
 
-  {% unless flag?(:win32) %}
+  {% unless flag?(:win32) || flag?(:wasm32) %}
     # Boehm GC requires to use GC_pthread_create and GC_pthread_join instead of pthread_create and pthread_join
     fun pthread_create = GC_pthread_create(thread : LibC::PthreadT*, attr : LibC::PthreadAttrT*, start : Void* -> Void*, arg : Void*) : LibC::Int
     fun pthread_join = GC_pthread_join(thread : LibC::PthreadT, value : Void**) : LibC::Int
diff --git a/src/gc/none.cr b/src/gc/none.cr
index 99ffa9d30b62..1937322fb488 100644
--- a/src/gc/none.cr
+++ b/src/gc/none.cr
@@ -60,7 +60,7 @@ module GC
       reclaimed_bytes_before_gc: zero)
   end
 
-  {% unless flag?(:win32) %}
+  {% unless flag?(:win32) || flag?(:wasm32) %}
     # :nodoc:
     def self.pthread_create(thread : LibC::PthreadT*, attr : LibC::PthreadAttrT*, start : Void* -> Void*, arg : Void*)
       LibC.pthread_create(thread, attr, start, arg)
diff --git a/src/io/file_descriptor.cr b/src/io/file_descriptor.cr
index 240bfab1084f..d6a57a72fb9c 100644
--- a/src/io/file_descriptor.cr
+++ b/src/io/file_descriptor.cr
@@ -25,7 +25,7 @@ class IO::FileDescriptor < IO
         end
     end
 
-    unless blocking || {{flag?(:win32)}}
+    unless blocking || {{ flag?(:win32) || flag?(:wasi) }}
       self.blocking = false
     end
   end
diff --git a/src/kernel.cr b/src/kernel.cr
index 94215527f753..9f63d2ffc678 100644
--- a/src/kernel.cr
+++ b/src/kernel.cr
@@ -502,7 +502,7 @@ def abort(message = nil, status = 1) : NoReturn
   exit status
 end
 
-{% unless flag?(:preview_mt) %}
+{% unless flag?(:preview_mt) || flag?(:wasm32) %}
   class Process
     # :nodoc:
     #
@@ -523,7 +523,7 @@ end
   end
 {% end %}
 
-{% unless flag?(:interpreted) %}
+{% unless flag?(:interpreted) || flag?(:wasm32) %}
   {% unless flag?(:win32) %}
     # Background loop to cleanup unused fiber stacks.
     spawn(name: "Fiber Clean Loop") do
diff --git a/src/lib_c/wasm32-wasi/c/arpa/inet.cr b/src/lib_c/wasm32-wasi/c/arpa/inet.cr
new file mode 100644
index 000000000000..afac8795f66f
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/arpa/inet.cr
@@ -0,0 +1,9 @@
+require "../netinet/in"
+require "../stdint"
+
+lib LibC
+  fun htons(x0 : UInt16T) : UInt16T
+  fun ntohs(x0 : UInt16T) : UInt16T
+  fun inet_ntop(x0 : Int, x1 : Void*, x2 : Char*, x3 : SocklenT) : Char*
+  fun inet_pton(x0 : Int, x1 : Char*, x2 : Void*) : Int
+end
diff --git a/src/lib_c/wasm32-wasi/c/errno.cr b/src/lib_c/wasm32-wasi/c/errno.cr
new file mode 100644
index 000000000000..662c355ec1d3
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/errno.cr
@@ -0,0 +1,79 @@
+lib LibC
+  E2BIG           =  1_u16
+  EACCES          =  2_u16
+  EADDRINUSE      =  3_u16
+  EADDRNOTAVAIL   =  4_u16
+  EAFNOSUPPORT    =  5_u16
+  EAGAIN          =  6_u16
+  EALREADY        =  7_u16
+  EBADF           =  8_u16
+  EBADMSG         =  9_u16
+  EBUSY           = 10_u16
+  ECANCELED       = 11_u16
+  ECHILD          = 12_u16
+  ECONNABORTED    = 13_u16
+  ECONNREFUSED    = 14_u16
+  ECONNRESET      = 15_u16
+  EDEADLK         = 16_u16
+  EDESTADDRREQ    = 17_u16
+  EDOM            = 18_u16
+  EDQUOT          = 19_u16
+  EEXIST          = 20_u16
+  EFAULT          = 21_u16
+  EFBIG           = 22_u16
+  EHOSTUNREACH    = 23_u16
+  EIDRM           = 24_u16
+  EILSEQ          = 25_u16
+  EINPROGRESS     = 26_u16
+  EINTR           = 27_u16
+  EINVAL          = 28_u16
+  EIO             = 29_u16
+  EISCONN         = 30_u16
+  EISDIR          = 31_u16
+  ELOOP           = 32_u16
+  EMFILE          = 33_u16
+  EMLINK          = 34_u16
+  EMSGSIZE        = 35_u16
+  EMULTIHOP       = 36_u16
+  ENAMETOOLONG    = 37_u16
+  ENETDOWN        = 38_u16
+  ENETRESET       = 39_u16
+  ENETUNREACH     = 40_u16
+  ENFILE          = 41_u16
+  ENOBUFS         = 42_u16
+  ENODEV          = 43_u16
+  ENOENT          = 44_u16
+  ENOEXEC         = 45_u16
+  ENOLCK          = 46_u16
+  ENOLINK         = 47_u16
+  ENOMEM          = 48_u16
+  ENOMSG          = 49_u16
+  ENOPROTOOPT     = 50_u16
+  ENOSPC          = 51_u16
+  ENOSYS          = 52_u16
+  ENOTCONN        = 53_u16
+  ENOTDIR         = 54_u16
+  ENOTEMPTY       = 55_u16
+  ENOTRECOVERABLE = 56_u16
+  ENOTSOCK        = 57_u16
+  ENOTSUP         = 58_u16
+  ENOTTY          = 59_u16
+  ENXIO           = 60_u16
+  EOPNOTSUPP      = LibC::ENOTSUP
+  EOVERFLOW       = 61_u16
+  EOWNERDEAD      = 62_u16
+  EPERM           = 63_u16
+  EPIPE           = 64_u16
+  EPROTO          = 65_u16
+  EPROTONOSUPPORT = 66_u16
+  EPROTOTYPE      = 67_u16
+  ERANGE          = 68_u16
+  EROFS           = 69_u16
+  ESPIPE          = 70_u16
+  ESRCH           = 71_u16
+  ESTALE          = 72_u16
+  ETIMEDOUT       = 73_u16
+  ETXTBSY         = 74_u16
+  EWOULDBLOCK     = LibC::EAGAIN
+  EXDEV           = 75_u16
+end
diff --git a/src/lib_c/wasm32-wasi/c/fcntl.cr b/src/lib_c/wasm32-wasi/c/fcntl.cr
new file mode 100644
index 000000000000..7bbfe10f23ec
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/fcntl.cr
@@ -0,0 +1,24 @@
+require "./sys/types"
+require "./sys/stat"
+require "./unistd"
+
+lib LibC
+  F_GETFD    = 1
+  F_SETFD    = 2
+  F_GETFL    = 3
+  F_SETFL    = 4
+  FD_CLOEXEC = 1
+  O_CLOEXEC  = 0
+  O_CREAT    = 1_u16 << 12
+  O_NOFOLLOW = 0x01000000
+  O_TRUNC    = 8_u16 << 12
+  O_APPEND   =      1_u16
+  O_NONBLOCK =      4_u16
+  O_SYNC     =     16_u16
+  O_RDONLY   = 0x04000000
+  O_RDWR     = O_RDONLY | O_WRONLY
+  O_WRONLY   = 0x10000000
+
+  fun fcntl(x0 : Int, x1 : Int, ...) : Int
+  fun open(x0 : Char*, x1 : Int, ...) : Int
+end
diff --git a/src/lib_c/wasm32-wasi/c/iconv.cr b/src/lib_c/wasm32-wasi/c/iconv.cr
new file mode 100644
index 000000000000..6496b957ed24
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/iconv.cr
@@ -0,0 +1,9 @@
+require "./stddef"
+
+lib LibC
+  type IconvT = Void*
+
+  fun iconv(x0 : IconvT, x1 : Char**, x2 : SizeT*, x3 : Char**, x4 : SizeT*) : SizeT
+  fun iconv_close(x0 : IconvT) : Int
+  fun iconv_open(x0 : Char*, x1 : Char*) : IconvT
+end
diff --git a/src/lib_c/wasm32-wasi/c/netdb.cr b/src/lib_c/wasm32-wasi/c/netdb.cr
new file mode 100644
index 000000000000..f73708969962
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/netdb.cr
@@ -0,0 +1,16 @@
+require "./netinet/in"
+require "./sys/socket"
+require "./stdint"
+
+lib LibC
+  struct Addrinfo
+    ai_flags : Int
+    ai_family : Int
+    ai_socktype : Int
+    ai_protocol : Int
+    ai_addrlen : SocklenT
+    ai_addr : Sockaddr*
+    ai_canonname : Char*
+    ai_next : Addrinfo*
+  end
+end
diff --git a/src/lib_c/wasm32-wasi/c/netinet/in.cr b/src/lib_c/wasm32-wasi/c/netinet/in.cr
new file mode 100644
index 000000000000..20a96d5ed22d
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/netinet/in.cr
@@ -0,0 +1,61 @@
+require "../sys/socket"
+require "../stdint"
+
+lib LibC
+  IPPROTO_IP   =   0
+  IPPROTO_IPV6 =  41
+  IPPROTO_ICMP =   1
+  IPPROTO_RAW  = 255
+  IPPROTO_TCP  =   6
+  IPPROTO_UDP  =  17
+
+  alias InPortT = UShort
+  alias InAddrT = UInt
+
+  struct InAddr
+    s_addr : InAddrT
+  end
+
+  struct In6Addr
+    s6_addr : StaticArray(UChar, 16)
+  end
+
+  struct SockaddrIn
+    sin_family : SaFamilyT
+    sin_port : InPortT
+    sin_addr : InAddr
+  end
+
+  struct SockaddrIn6
+    sin6_family : SaFamilyT
+    sin6_port : InPortT
+    sin6_flowinfo : UInt
+    sin6_addr : In6Addr
+    sin6_scope_id : UInt
+  end
+
+  IP_MULTICAST_IF   = 32
+  IPV6_MULTICAST_IF = 17
+
+  IP_MULTICAST_TTL    = 33
+  IPV6_MULTICAST_HOPS = 18
+
+  IP_MULTICAST_LOOP   = 34
+  IPV6_MULTICAST_LOOP = 19
+
+  IP_ADD_MEMBERSHIP = 35
+  IPV6_JOIN_GROUP   = 20
+
+  IP_DROP_MEMBERSHIP = 36
+  IPV6_LEAVE_GROUP   = 21
+
+  struct IpMreq
+    imr_multiaddr : InAddr
+    imr_interface : InAddr
+  end
+
+  struct Ipv6Mreq
+    ipv6mr_multiaddr : In6Addr
+    ipv6mr_interface : UInt
+  end
+end
diff --git a/src/lib_c/wasm32-wasi/c/netinet/tcp.cr b/src/lib_c/wasm32-wasi/c/netinet/tcp.cr
new file mode 100644
index 000000000000..beed1db6e081
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/netinet/tcp.cr
@@ -0,0 +1,6 @@
+lib LibC
+  TCP_NODELAY   = 1
+  TCP_KEEPIDLE  = 4
+  TCP_KEEPINTVL = 5
+  TCP_KEEPCNT   = 6
+end
diff --git a/src/lib_c/wasm32-wasi/c/sched.cr b/src/lib_c/wasm32-wasi/c/sched.cr
new file mode 100644
index 000000000000..9d83ed18504e
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/sched.cr
@@ -0,0 +1,3 @@
+lib LibC
+  fun sched_yield : Int
+end
diff --git a/src/lib_c/wasm32-wasi/c/stdarg.cr b/src/lib_c/wasm32-wasi/c/stdarg.cr
new file mode 100644
index 000000000000..882d4f51d35c
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/stdarg.cr
@@ -0,0 +1,3 @@
+lib LibC
+  type VaList = Void*
+end
diff --git a/src/lib_c/wasm32-wasi/c/stddef.cr b/src/lib_c/wasm32-wasi/c/stddef.cr
new file mode 100644
index 000000000000..4afcdf34d723
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/stddef.cr
@@ -0,0 +1,3 @@
+lib LibC
+  alias SizeT = ULong
+end
diff --git a/src/lib_c/wasm32-wasi/c/stdint.cr b/src/lib_c/wasm32-wasi/c/stdint.cr
new file mode 100644
index 000000000000..c3d333b17104
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/stdint.cr
@@ -0,0 +1,10 @@
+lib LibC
+  alias Int8T = SChar
+  alias Int16T = Short
+  alias Int32T = Int
+  alias Int64T = LongLong
+  alias UInt8T = UChar
+  alias UInt16T = UShort
+  alias UInt32T = UInt
+  alias UInt64T = ULongLong
+end
diff --git a/src/lib_c/wasm32-wasi/c/stdio.cr b/src/lib_c/wasm32-wasi/c/stdio.cr
new file mode 100644
index 000000000000..98967c3ddfe9
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/stdio.cr
@@ -0,0 +1,9 @@
+require "./sys/types"
+require "./stddef"
+
+lib LibC
+  fun dprintf(x0 : Int, x1 : Char*, ...) : Int
+  fun printf(x0 : Char*, ...) : Int
+  fun rename(x0 : Char*, x1 : Char*) : Int
+  fun snprintf(x0 : Char*, x1 : SizeT, x2 : Char*, ...) : Int
+end
diff --git a/src/lib_c/wasm32-wasi/c/stdlib.cr b/src/lib_c/wasm32-wasi/c/stdlib.cr
new file mode 100644
index 000000000000..619175b50d05
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/stdlib.cr
@@ -0,0 +1,25 @@
+require "./stddef"
+
+lib LibC
+  struct DivT
+    quot : Int
+    rem : Int
+  end
+
+  fun arc4random : UInt32T
+  fun arc4random_buf(x0 : Void*, x1 : SizeT) : Void
+  fun arc4random_uniform(x0 : UInt32T) : UInt32T
+  fun atof(x0 : Char*) : Double
+  fun div(x0 : Int, x1 : Int) : DivT
+  fun exit(x0 : Int) : NoReturn
+  fun free(ptr : Void*) : Void
+  fun getenv(x0 : Char*) : Char*
+  fun malloc(size : SizeT) : Void*
+  fun putenv(x0 : Char*) : Int
+  fun realloc(ptr : Void*, size : SizeT) : Void*
+  fun setenv(x0 : Char*, x1 : Char*, x2 : Int) : Int
+  fun strtod(x0 : Char*, x1 : Char**) : Double
+  fun strtof(x0 : Char*, x1 : Char**) : Float
+  fun strtol(x0 : Char*, x1 : Char**, x2 : Int) : Long
+  fun unsetenv(x0 : Char*) : Int
+end
diff --git a/src/lib_c/wasm32-wasi/c/string.cr b/src/lib_c/wasm32-wasi/c/string.cr
new file mode 100644
index 000000000000..5be77e03cf1c
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/string.cr
@@ -0,0 +1,9 @@
+require "./stddef"
+
+lib LibC
+  fun memchr(x0 : Void*, x1 : Int, x2 : SizeT) : Void*
+  fun memcmp(x0 : Void*, x1 : Void*, x2 : SizeT) : Int
+  fun strcmp(x0 : Char*, x1 : Char*) : Int
+  fun strerror(x0 : Int) : Char*
+  fun strlen(x0 : Char*) : ULong
+end
diff --git a/src/lib_c/wasm32-wasi/c/sys/file.cr b/src/lib_c/wasm32-wasi/c/sys/file.cr
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/src/lib_c/wasm32-wasi/c/sys/resource.cr b/src/lib_c/wasm32-wasi/c/sys/resource.cr
new file mode 100644
index 000000000000..655a9009d3a9
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/sys/resource.cr
@@ -0,0 +1,11 @@
+lib LibC
+  RUSAGE_SELF     = 1
+  RUSAGE_CHILDREN = 2
+
+  struct Rusage
+    ru_utime : Timeval
+    ru_stime : Timeval
+  end
+
+  fun getrusage(who : Int, usage : Rusage*) : Int
+end
diff --git a/src/lib_c/wasm32-wasi/c/sys/select.cr b/src/lib_c/wasm32-wasi/c/sys/select.cr
new file mode 100644
index 000000000000..ef8a031287a7
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/sys/select.cr
@@ -0,0 +1,13 @@
+require "./types"
+require "./time"
+require "../time"
+require "../signal"
+
+lib LibC
+  struct FdSet
+    __nfds : SizeT
+    __fds : StaticArray(Int, 1024)
+  end
+
+  fun select(x0 : Int, x1 : FdSet*, x2 : FdSet*, x3 : FdSet*, x4 : Timeval*) : Int
+end
diff --git a/src/lib_c/wasm32-wasi/c/sys/socket.cr b/src/lib_c/wasm32-wasi/c/sys/socket.cr
new file mode 100644
index 000000000000..82bcec46e9cc
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/sys/socket.cr
@@ -0,0 +1,38 @@
+require "./types"
+
+lib LibC
+  SOCK_DGRAM   =       5_u8
+  SOCK_STREAM  =       6_u8
+  SOL_SOCKET   = 0x7fffffff
+  AF_INET      =          1
+  AF_INET6     =          2
+  AF_UNIX      =          3
+  AF_UNSPEC    =          0
+  SHUT_RD      =       1_u8
+  SHUT_RDWR    = SHUT_RD | SHUT_WR
+  SHUT_WR      =       2_u8
+  SOCK_CLOEXEC = 0x00002000
+
+  alias SocklenT = UInt
+  alias SaFamilyT = UShort
+
+  struct Sockaddr
+    sa_family : SaFamilyT
+    sa_data : StaticArray(Char, 0)
+  end
+
+  struct SockaddrStorage
+    ss_family : SaFamilyT
+    __ss_data : StaticArray(Char, 32)
+  end
+
+  struct Linger
+    l_onoff : Int
+    l_linger : Int
+  end
+
+  fun getsockopt(x0 : Int, x1 : Int, x2 : Int, x3 : Void*, x4 : SocklenT*) : Int
+  fun recv(x0 : Int, x1 : Void*, x2 : SizeT, x3 : Int) : SSizeT
+  fun send(x0 : Int, x1 : Void*, x2 : SizeT, x3 : Int) : SSizeT
+  fun shutdown(x0 : Int, x1 : Int) : Int
+end
diff --git a/src/lib_c/wasm32-wasi/c/sys/stat.cr b/src/lib_c/wasm32-wasi/c/sys/stat.cr
new file mode 100644
index 000000000000..5b40bb7850bd
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/sys/stat.cr
@@ -0,0 +1,51 @@
+require "./types"
+require "../time"
+
+lib LibC
+  S_IFMT   = S_IFBLK | S_IFCHR | S_IFDIR | S_IFIFO | S_IFLNK | S_IFREG | S_IFSOCK
+  S_IFBLK  = 0x6000
+  S_IFCHR  = 0x2000
+  S_IFIFO  = 0xc000
+  S_IFREG  = 0x8000
+  S_IFDIR  = 0x4000
+  S_IFLNK  = 0xa000
+  S_IFSOCK = 0xc000
+  S_IRUSR  =  0x100
+  S_IWUSR  =   0x80
+  S_IXUSR  =   0x40
+  S_IRWXU  = S_IXUSR | S_IWUSR | S_IRUSR
+  S_IRGRP  = 0x20
+  S_IWGRP  = 0x10
+  S_IXGRP  =  0x8
+  S_IRWXG  = S_IXGRP | S_IWGRP | S_IRGRP
+  S_IROTH  = 0x4
+  S_IWOTH  = 0x2
+  S_IXOTH  = 0x1
+  S_IRWXO  = S_IXOTH | S_IWOTH | S_IROTH
+  S_ISUID  = 0x800
+  S_ISGID  = 0x400
+  S_ISVTX  = 0x200
+
+  struct Stat
+    st_dev : DevT
+    st_ino : InoT
+    st_nlink : NlinkT
+    st_mode : ModeT
+    st_uid : UidT
+    st_gid : GidT
+    __pad0 : UInt
+    st_rdev : DevT
+    st_size : OffT
+    st_blksize : BlksizeT
+    st_blocks : BlkcntT
+    st_atim : Timespec
+    st_mtim : Timespec
+    st_ctim : Timespec
+    __reserved : StaticArray(LongLong, 3)
+  end
+
+  fun fstat(x0 : Int, x1 : Stat*) : Int
+  fun lstat(x0 : Char*, x1 : Stat*) : Int
+  fun mkdir(x0 : Char*, x1 : ModeT) : Int
+  fun stat(x0 : Char*, x1 : Stat*) : Int
+end
diff --git a/src/lib_c/wasm32-wasi/c/sys/time.cr b/src/lib_c/wasm32-wasi/c/sys/time.cr
new file mode 100644
index 000000000000..2f0678b910c8
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/sys/time.cr
@@ -0,0 +1,15 @@
+require "./types"
+
+lib LibC
+  struct Timeval
+    tv_sec : TimeT
+    tv_usec : SusecondsT
+  end
+
+  struct Timezone
+    tz_minuteswest : Int
+    tz_dsttime : Int
+  end
+
+  fun gettimeofday(x0 : Timeval*, x1 : Void*) : Int
+end
diff --git a/src/lib_c/wasm32-wasi/c/sys/times.cr b/src/lib_c/wasm32-wasi/c/sys/times.cr
new file mode 100644
index 000000000000..c190774559f4
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/sys/times.cr
@@ -0,0 +1,12 @@
+require "./types"
+
+lib LibC
+  struct Tms
+    tms_utime : ClockT
+    tms_stime : ClockT
+    tms_cutime : ClockT
+    tms_cstime : ClockT
+  end
+
+  fun times(x0 : Tms*) : ClockT
+end
diff --git a/src/lib_c/wasm32-wasi/c/sys/types.cr b/src/lib_c/wasm32-wasi/c/sys/types.cr
new file mode 100644
index 000000000000..5cca3a65b89b
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/sys/types.cr
@@ -0,0 +1,61 @@
+require "../stddef"
+require "../stdint"
+
+lib LibC
+  alias BlkcntT = LongLong
+  alias BlksizeT = Long
+  alias ClockT = LongLong
+  alias ClockidT = Int
+  alias DevT = ULongLong
+  alias GidT = UInt
+  alias IdT = UInt
+  alias InoT = ULongLong
+  alias ModeT = UInt
+  alias NlinkT = ULongLong
+  alias OffT = LongLong
+  alias PidT = Int
+
+  union PthreadAttrTU
+    __i : StaticArray(Int, 14)
+    __vi : StaticArray(Int, 14)
+    __s : StaticArray(ULong, 7)
+  end
+
+  struct PthreadAttrT
+    __u : PthreadAttrTU
+  end
+
+  union PthreadCondTU
+    __i : StaticArray(Int, 12)
+    __vi : StaticArray(Int, 12)
+    __p : StaticArray(Void*, 6)
+  end
+
+  struct PthreadCondT
+    __u : PthreadCondTU
+  end
+
+  struct PthreadCondattrT
+    __attr : UInt
+  end
+
+  union PthreadMutexTU
+    __i : StaticArray(Int, 10)
+    __vi : StaticArray(Int, 10)
+    __p : StaticArray(Void*, 5)
+  end
+
+  struct PthreadMutexT
+    __u : PthreadMutexTU
+  end
+
+  struct PthreadMutexattrT
+    __attr : UInt
+  end
+
+  type PthreadT = Void*
+  alias SSizeT = Long
+  alias SusecondsT = LongLong
+  alias TimeT = LongLong
+  alias UidT = UInt
+end
diff --git a/src/lib_c/wasm32-wasi/c/termios.cr b/src/lib_c/wasm32-wasi/c/termios.cr
new file mode 100644
index 000000000000..be54a30c3caf
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/termios.cr
@@ -0,0 +1,120 @@
+require "./sys/types"
+
+lib LibC
+  VEOF      =        4
+  VEOL      =       11
+  VERASE    =        2
+  VINTR     =        0
+  VKILL     =        3
+  VMIN      =        6
+  VQUIT     =        1
+  VSTART    =        8
+  VSTOP     =        9
+  VSUSP     =       10
+  BRKINT    = 0o000002
+  ICRNL     = 0o000400
+  IGNBRK    = 0o000001
+  IGNCR     = 0o000200
+  IGNPAR    = 0o000004
+  INLCR     = 0o000100
+  INPCK     = 0o000020
+  ISTRIP    = 0o000040
+  IXANY     = 0o004000
+  IXOFF     = 0o010000
+  IXON      = 0o002000
+  PARMRK    = 0o000010
+  OPOST     = 0o000001
+  ONLCR     = 0o000004
+  OCRNL     = 0o000010
+  ONOCR     = 0o000020
+  ONLRET    = 0o000040
+  OFDEL     = 0o000200
+  OFILL     = 0o000100
+  CRDLY     = 0o003000
+  CR0       = 0o000000
+  CR1       = 0o001000
+  CR2       = 0o002000
+  CR3       = 0o003000
+  TABDLY    = 0o014000
+  TAB0      = 0o000000
+  TAB1      = 0o004000
+  TAB2      = 0o010000
+  TAB3      = 0o014000
+  BSDLY     = 0o020000
+  BS0       = 0o000000
+  BS1       = 0o020000
+  VTDLY     = 0o040000
+  VT0       = 0o000000
+  VT1       = 0o040000
+  FFDLY     = 0o100000
+  FF0       = 0o000000
+  FF1       = 0o100000
+  NLDLY     = 0o000400
+  NL0       = 0o000000
+  NL1       = 0o000400
+  B0        = 0o000000
+  B50       = 0o000001
+  B75       = 0o000002
+  B110      = 0o000003
+  B134      = 0o000004
+  B150      = 0o000005
+  B200      = 0o000006
+  B300      = 0o000007
+  B600      = 0o000010
+  B1200     = 0o000011
+  B1800     = 0o000012
+  B2400     = 0o000013
+  B4800     = 0o000014
+  B9600     = 0o000015
+  B19200    = 0o000016
+  B38400    = 0o000017
+  CSIZE     = 0o000060
+  CS5       = 0o000000
+  CS6       = 0o000020
+  CS7       = 0o000040
+  CS8       = 0o000060
+  CSTOPB    = 0o000100
+  CREAD     = 0o000200
+  PARENB    = 0o000400
+  PARODD    = 0o001000
+  HUPCL     = 0o002000
+  CLOCAL    = 0o004000
+  ECHO      = 0o000010
+  ECHOE     = 0o000020
+  ECHOK     = 0o000040
+  ECHONL    = 0o000100
+  ICANON    = 0o000002
+  IEXTEN    = 0o100000
+  ISIG      = 0o000001
+  NOFLSH    = 0o000200
+  TOSTOP    = 0o000400
+  TCSANOW   =        0
+  TCSADRAIN =        1
+  TCSAFLUSH =        2
+  TCIFLUSH  =        0
+  TCIOFLUSH =        2
+  TCOFLUSH  =        1
+  TCIOFF    =        2
+  TCION     =        3
+  TCOOFF    =        0
+  TCOON     =        1
+
+  alias CcT = UInt8
+  alias SpeedT = UInt
+  alias TcflagT = UInt
+
+  struct Termios
+    c_iflag : TcflagT
+    c_oflag : TcflagT
+    c_cflag : TcflagT
+    c_lflag : TcflagT
+    c_line : CcT
+    c_cc : StaticArray(CcT, 32)
+    __c_ispeed : SpeedT
+    __c_ospeed : SpeedT
+  end
+
+  fun tcgetattr(x0 : Int, x1 : Termios*) : Int
+  fun tcsetattr(x0 : Int, x1 : Int, x2 : Termios*) : Int
+  fun cfmakeraw(x0 : Termios*) : Void
+end
diff --git a/src/lib_c/wasm32-wasi/c/time.cr b/src/lib_c/wasm32-wasi/c/time.cr
new file mode 100644
index 000000000000..6b8fc35a21eb
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/time.cr
@@ -0,0 +1,32 @@
+require "./sys/types"
+
+lib LibC
+  CLOCK_MONOTONIC = 1
+  CLOCK_REALTIME  = 0
+
+  struct Tm
+    tm_sec : Int
+    tm_min : Int
+    tm_hour : Int
+    tm_mday : Int
+    tm_mon : Int
+    tm_year : Int
+    tm_wday : Int
+    tm_yday : Int
+    tm_isdst : Int
+    tm_gmtoff : Int
+    tm_zone : Char*
+    __tm_nsec : Int
+  end
+
+  struct Timespec
+    tv_sec : TimeT
+    tv_nsec : Long
+  end
+
+  fun clock_gettime(x0 : ClockidT, x1 : Timespec*) : Int
+  fun gmtime_r(x0 : TimeT*, x1 : Tm*) : Tm*
+  fun localtime_r(x0 : TimeT*, x1 : Tm*) : Tm*
+  fun mktime(x0 : Tm*) : TimeT
+  fun timegm(x0 : Tm*) : TimeT
+end
diff --git a/src/lib_c/wasm32-wasi/c/unistd.cr b/src/lib_c/wasm32-wasi/c/unistd.cr
new file mode 100644
index 000000000000..7b4a5949cafd
--- /dev/null
+++ b/src/lib_c/wasm32-wasi/c/unistd.cr
@@ -0,0 +1,32 @@
+require "./sys/types"
+require "./stdint"
+
+lib LibC
+  F_OK                =  0
+  R_OK                =  1
+  W_OK                =  2
+  X_OK                =  4
+  SC_CLK_TCK          =  2
+  SC_NPROCESSORS_ONLN = 84
+
+  fun access(x0 : Char*, x1 : Int) : Int
+  fun chdir(x0 : Char*) : Int
+  fun close(fd : Int) : Int
+  fun _exit(x0 : Int) : NoReturn
+  fun fdatasync(x0 : Int) : Int
+  fun fsync(x0 : Int) : Int
+  fun ftruncate(x0 : Int, x1 : OffT) : Int
+  fun getcwd(x0 : Char*, x1 : SizeT) : Char*
+  fun isatty(x0 : Int) : Int
+  fun link(x0 : Char*, x1 : Char*) : Int
+  fun lseek(x0 : Int, x1 : OffT, x2 : Int) : OffT
+  fun read(x0 : Int, x1 : Void*, x2 : SizeT) : SSizeT
+  fun pread(x0 : Int, x1 : Void*, x2 : SizeT, x3 : OffT) : SSizeT
+  fun readlink(x0 : Char*, x1 : Char*, x2 : SizeT) : SSizeT
+  fun rmdir(x0 : Char*) : Int
+  fun sleep(x0 : UInt) : UInt
+  fun symlink(x0 : Char*, x1 : Char*) : Int
+  fun sysconf(x0 : Int) : Long
+  fun unlink(x0 : Char*) : Int
+  fun write(x0 : Int, x1 : Void*, x2 : SizeT) : SSizeT
+end
diff --git a/src/llvm.cr b/src/llvm.cr
index 9f0c388603b8..ab36cc57d966 100644
--- a/src/llvm.cr
+++ b/src/llvm.cr
@@ -55,6 +55,23 @@ module LLVM
     {% end %}
   end
 
+  def self.init_webassembly : Nil
+    return if @@initialized_webassembly
+    @@initialized_webassembly = true
+
+    {% if LibLLVM::BUILT_TARGETS.includes?(:webassembly) %}
+      LibLLVM.initialize_webassembly_target_info
+      LibLLVM.initialize_webassembly_target
+      LibLLVM.initialize_webassembly_target_mc
+      LibLLVM.initialize_webassembly_asm_printer
+      LibLLVM.initialize_webassembly_asm_parser
+      # LibLLVM.link_in_jit
+      LibLLVM.link_in_mc_jit
+    {% else %}
+      raise "ERROR: LLVM was built without WebAssembly target"
+    {% end %}
+  end
+
   def self.start_multithreaded : Bool
     if multithreaded?
       true
diff --git a/src/llvm/abi/wasm32.cr b/src/llvm/abi/wasm32.cr
new file mode 100644
index 000000000000..1882a44c8c4a
--- /dev/null
+++ b/src/llvm/abi/wasm32.cr
@@ -0,0 +1,44 @@
+require "../abi"
+
+class LLVM::ABI::Wasm32 < LLVM::ABI
+  def abi_info(atys : Array(Type), rty : Type, ret_def : Bool, context : Context)
+    ret_ty = compute_return_type(rty, ret_def, context)
+    arg_tys = compute_arg_types(atys, context)
+    FunctionType.new(arg_tys, ret_ty)
+  end
+
+  def align(type : Type)
+    target_data.abi_alignment(type).to_i32
+  end
+
+  def size(type : Type)
+    target_data.abi_size(type).to_i32
+  end
+
+  private def aggregate?(type)
+    case type.kind
+    when .struct?, .array?
+      true
+    else
+      false
+    end
+  end
+
+  private def compute_return_type(rty, ret_def, context)
+    if aggregate?(rty)
+      ArgType.indirect(rty, LLVM::Attribute::ByVal)
+    else
+      ArgType.direct(rty)
+    end
+  end
+
+  private def compute_arg_types(atys, context)
+    atys.map do |t|
+      if aggregate?(t)
+        ArgType.indirect(t, LLVM::Attribute::ByVal)
+      else
+        ArgType.direct(t)
+      end
+    end
+  end
+end
diff --git a/src/llvm/lib_llvm.cr b/src/llvm/lib_llvm.cr
index f633a9cf8277..f4a384e329df 100644
--- a/src/llvm/lib_llvm.cr
+++ b/src/llvm/lib_llvm.cr
@@ -225,6 +225,11 @@ lib LibLLVM
   fun initialize_arm_target = LLVMInitializeARMTarget
   fun initialize_arm_target_info = LLVMInitializeARMTargetInfo
   fun initialize_arm_target_mc = LLVMInitializeARMTargetMC
+  fun initialize_webassembly_asm_printer = LLVMInitializeWebAssemblyAsmPrinter
+  fun initialize_webassembly_asm_parser = LLVMInitializeWebAssemblyAsmParser
+  fun initialize_webassembly_target = LLVMInitializeWebAssemblyTarget
+  fun initialize_webassembly_target_info = LLVMInitializeWebAssemblyTargetInfo
+  fun initialize_webassembly_target_mc = LLVMInitializeWebAssemblyTargetMC
   fun initialize_native_target = LLVMInitializeNativeTarget
   fun is_constant = LLVMIsConstant(val : ValueRef) : Int32
   fun is_function_var_arg = LLVMIsFunctionVarArg(ty : TypeRef) : Int32
diff --git a/src/llvm/target_machine.cr b/src/llvm/target_machine.cr
index ee7a413df959..1b9dbf685921 100644
--- a/src/llvm/target_machine.cr
+++ b/src/llvm/target_machine.cr
@@ -57,6 +57,8 @@ class LLVM::TargetMachine
       ABI::AArch64.new(self)
     when /arm/
       ABI::ARM.new(self)
+    when /wasm32/
+      ABI::Wasm32.new(self)
     else
       raise "Unsupported ABI for target triple: #{triple}"
     end
diff --git a/src/log/io_backend.cr b/src/log/io_backend.cr
index cc7e38458552..9092495c8bf4 100644
--- a/src/log/io_backend.cr
+++ b/src/log/io_backend.cr
@@ -3,8 +3,8 @@ class Log::IOBackend < Log::Backend
   property io : IO
   property formatter : Formatter
 
-  {% if flag?(:win32) %}
-    # TODO: this constructor must go away once channels are fixed in Windows
+  {% if flag?(:win32) || flag?(:wasm32) %}
+    # TODO: this constructor must go away once channels are fixed in Windows / WebAssembly
     def initialize(@io = STDOUT, *, @formatter : Formatter = ShortFormat, dispatcher : Dispatcher::Spec = DispatchMode::Sync)
       super(dispatcher)
     end
diff --git a/src/prelude.cr b/src/prelude.cr
index 1b934882d374..ea88f25215f1 100644
--- a/src/prelude.cr
+++ b/src/prelude.cr
@@ -45,6 +45,7 @@ require "enumerable"
 require "env"
 require "errno"
 require "winerror"
+require "wasi_error"
 require "file"
 require "float"
 require "gc"
@@ -71,7 +72,7 @@ require "range"
 require "reference"
 require "regex"
 require "set"
-{% unless flag?(:win32) %}
+{% unless flag?(:win32) || flag?(:wasm32) %}
   require "signal"
 {% end %}
 require "slice"
diff --git a/src/process/status.cr b/src/process/status.cr
index 53c74006a1db..56da75515688 100644
--- a/src/process/status.cr
+++ b/src/process/status.cr
@@ -48,6 +48,13 @@ class Process::Status
     {% end %}
   end
 
+  {% if flag?(:wasm32) %}
+    # wasm32 does not define `Signal`
+    def exit_signal
+      raise NotImplementedError.new("Process::Status#exit_signal")
+    end
+  {% end %}
+
   # If `normal_exit?` is `true`, returns the exit code of the process.
   def exit_code : Int32
     {% if flag?(:unix) %}
diff --git a/src/raise.cr b/src/raise.cr
index 54d1d11157f1..30653a790662 100644
--- a/src/raise.cr
+++ b/src/raise.cr
@@ -170,6 +170,31 @@ end
 
     __crystal_continue_unwind
   end
+{% elsif flag?(:wasm32) %}
+  # :nodoc:
+  fun __crystal_personality
+    LibC.printf("EXITING: __crystal_personality called")
+    LibC.exit(1)
+  end
+
+  # :nodoc:
+  @[Raises]
+  fun __crystal_raise(ex : Void*) : NoReturn
+    LibC.printf("EXITING: __crystal_raise called")
+    LibC.exit(1)
+  end
+
+  # :nodoc:
+  fun __crystal_get_exception(ex : Void*) : UInt64
+    LibC.printf("EXITING: __crystal_get_exception called")
+    LibC.exit(1)
+    0u64
+  end
+
+  def raise(exception : Exception) : NoReturn
+    LibC.printf("EXITING: Attempting to raise:\n#{exception.inspect_with_backtrace}")
+    LibC.exit(1)
+  end
 {% else %}
   # :nodoc:
   fun __crystal_personality(version : Int32, actions : LibUnwind::Action, exception_class : UInt64, exception_object : LibUnwind::Exception*, context : Void*) : LibUnwind::ReasonCode
@@ -189,7 +214,7 @@ end
   end
 {% end %}
 
-{% unless flag?(:win32) %}
+{% unless flag?(:win32) || flag?(:wasm32) %}
   # :nodoc:
   @[Raises]
   fun __crystal_raise(unwind_ex : LibUnwind::Exception*) : NoReturn
diff --git a/src/socket.cr b/src/socket.cr
index f315681ee709..508f76f08e9b 100644
--- a/src/socket.cr
+++ b/src/socket.cr
@@ -264,55 +264,105 @@ class Socket < IO
     io << "#<#{self.class}:fd #{fd}>"
   end
 
-  def send_buffer_size : Int32
-    getsockopt LibC::SO_SNDBUF, 0
-  end
+  {% if flag?(:wasm32) %}
+    def send_buffer_size : Int32
+      raise NotImplementedError.new "Socket#send_buffer_size"
+    end
 
-  def send_buffer_size=(val : Int32)
-    setsockopt LibC::SO_SNDBUF, val
-    val
-  end
+    def send_buffer_size=(val : Int32)
+      raise NotImplementedError.new "Socket#send_buffer_size="
+    end
 
-  def recv_buffer_size : Int32
-    getsockopt LibC::SO_RCVBUF, 0
-  end
+    def recv_buffer_size : Int32
+      raise NotImplementedError.new "Socket#recv_buffer_size"
+    end
 
-  def recv_buffer_size=(val : Int32)
-    setsockopt LibC::SO_RCVBUF, val
-    val
-  end
+    def recv_buffer_size=(val : Int32)
+      raise NotImplementedError.new "Socket#recv_buffer_size="
+    end
 
-  def reuse_address? : Bool
-    getsockopt_bool LibC::SO_REUSEADDR
-  end
+    def reuse_address? : Bool
+      raise NotImplementedError.new "Socket#reuse_address?"
+    end
 
-  def reuse_address=(val : Bool)
-    setsockopt_bool LibC::SO_REUSEADDR, val
-  end
+    def reuse_address=(val : Bool)
+      raise NotImplementedError.new "Socket#reuse_address="
+    end
 
-  def reuse_port? : Bool
-    system_reuse_port?
-  end
+    def reuse_port? : Bool
+      raise NotImplementedError.new "Socket#reuse_port?"
+    end
 
-  def reuse_port=(val : Bool)
-    self.system_reuse_port = val
-  end
+    def reuse_port=(val : Bool)
+      raise NotImplementedError.new "Socket#reuse_port="
+    end
 
-  def broadcast? : Bool
-    getsockopt_bool LibC::SO_BROADCAST
-  end
+    def broadcast? : Bool
+      raise NotImplementedError.new "Socket#broadcast?"
+    end
 
-  def broadcast=(val : Bool)
-    setsockopt_bool LibC::SO_BROADCAST, val
-  end
+    def broadcast=(val : Bool)
+      raise NotImplementedError.new "Socket#broadcast="
+    end
 
-  def keepalive?
-    getsockopt_bool LibC::SO_KEEPALIVE
-  end
+    def keepalive?
+      raise NotImplementedError.new "Socket#keepalive?"
+    end
 
-  def keepalive=(val : Bool)
-    setsockopt_bool LibC::SO_KEEPALIVE, val
-  end
+    def keepalive=(val : Bool)
+      raise NotImplementedError.new "Socket#keepalive="
+    end
+  {% else %}
+    def send_buffer_size : Int32
+      getsockopt LibC::SO_SNDBUF, 0
+    end
+
+    def send_buffer_size=(val : Int32)
+      setsockopt LibC::SO_SNDBUF, val
+      val
+    end
+
+    def recv_buffer_size : Int32
+      getsockopt LibC::SO_RCVBUF, 0
+    end
+
+    def recv_buffer_size=(val : Int32)
+      setsockopt LibC::SO_RCVBUF, val
+      val
+    end
+
+    def reuse_address? : Bool
+      getsockopt_bool LibC::SO_REUSEADDR
+    end
+
+    def reuse_address=(val : Bool)
+      setsockopt_bool LibC::SO_REUSEADDR, val
+    end
+
+    def reuse_port? : Bool
+      system_reuse_port?
+    end
+
+    def reuse_port=(val : Bool)
+      self.system_reuse_port = val
+    end
+
+    def broadcast? : Bool
+      getsockopt_bool LibC::SO_BROADCAST
+    end
+
+    def broadcast=(val : Bool)
+      setsockopt_bool LibC::SO_BROADCAST, val
+    end
+
+    def keepalive?
+      getsockopt_bool LibC::SO_KEEPALIVE
+    end
+
+    def keepalive=(val : Bool)
+      setsockopt_bool LibC::SO_KEEPALIVE, val
+    end
+  {% end %}
 
   def linger
     system_linger
diff --git a/src/socket/address.cr b/src/socket/address.cr
index f1279e63cf59..2ecfc32de029 100644
--- a/src/socket/address.cr
+++ b/src/socket/address.cr
@@ -244,6 +244,8 @@ class Socket
         addr.__u6_addr.__u6_addr8
       {% elsif flag?(:linux) && flag?(:musl) %}
         addr.__in6_union.__s6_addr
+      {% elsif flag?(:wasm32) %}
+        addr.s6_addr
       {% elsif flag?(:linux) %}
         addr.__in6_u.__u6_addr8
       {% elsif flag?(:win32) %}
@@ -329,19 +331,31 @@ class Socket
     getter path : String
 
     # :nodoc:
-    MAX_PATH_SIZE = LibC::SockaddrUn.new.sun_path.size - 1
+    MAX_PATH_SIZE = {% if flag?(:wasm32) %}
+                      0
+                    {% else %}
+                      LibC::SockaddrUn.new.sun_path.size - 1
+                    {% end %}
 
     def initialize(@path : String)
       if @path.bytesize + 1 > MAX_PATH_SIZE
         raise ArgumentError.new("Path size exceeds the maximum size of #{MAX_PATH_SIZE} bytes")
       end
       @family = Family::UNIX
-      @size = sizeof(LibC::SockaddrUn)
+      @size = {% if flag?(:wasm32) %}
+                1
+              {% else %}
+                sizeof(LibC::SockaddrUn)
+              {% end %}
     end
 
     # Creates an `UNIXSocket` from the internal OS representation.
     def self.from(sockaddr : LibC::Sockaddr*, addrlen) : UNIXAddress
-      new(sockaddr.as(LibC::SockaddrUn*), addrlen.to_i)
+      {% if flag?(:wasm32) %}
+        raise NotImplementedError.new "Socket::UnixAddress.from"
+      {% else %}
+        new(sockaddr.as(LibC::SockaddrUn*), addrlen.to_i)
+      {% end %}
     end
 
     # Parses a `Socket::UNIXAddress` from an URI.
@@ -383,11 +397,13 @@ class Socket
       parse URI.parse(uri)
     end
 
-    protected def initialize(sockaddr : LibC::SockaddrUn*, size)
-      @family = Family::UNIX
-      @path = String.new(sockaddr.value.sun_path.to_unsafe)
-      @size = size || sizeof(LibC::SockaddrUn)
-    end
+    {% unless flag?(:wasm32) %}
+      protected def initialize(sockaddr : LibC::SockaddrUn*, size)
+        @family = Family::UNIX
+        @path = String.new(sockaddr.value.sun_path.to_unsafe)
+        @size = size || sizeof(LibC::SockaddrUn)
+      end
+    {% end %}
 
     def_equals_and_hash path
 
@@ -396,10 +412,14 @@ class Socket
     end
 
     def to_unsafe : LibC::Sockaddr*
-      sockaddr = Pointer(LibC::SockaddrUn).malloc
-      sockaddr.value.sun_family = family
-      sockaddr.value.sun_path.to_unsafe.copy_from(@path.to_unsafe, @path.bytesize + 1)
-      sockaddr.as(LibC::Sockaddr*)
+      {% if flag?(:wasm32) %}
+        raise NotImplementedError.new "Socket::UnixAddress#to_unsafe"
+      {% else %}
+        sockaddr = Pointer(LibC::SockaddrUn).malloc
+        sockaddr.value.sun_family = family
+        sockaddr.value.sun_path.to_unsafe.copy_from(@path.to_unsafe, @path.bytesize + 1)
+        sockaddr.as(LibC::Sockaddr*)
+      {% end %}
     end
   end
 
diff --git a/src/socket/addrinfo.cr b/src/socket/addrinfo.cr
index 9163b6c9d617..8e6a43539fae 100644
--- a/src/socket/addrinfo.cr
+++ b/src/socket/addrinfo.cr
@@ -129,58 +129,62 @@ class Socket
     end
 
     private def self.getaddrinfo(domain, service, family, type, protocol, timeout)
-      # RFC 3986 says:
-      # > When a non-ASCII registered name represents an internationalized domain name
-      # > intended for resolution via the DNS, the name must be transformed to the IDNA
-      # > encoding [RFC3490] prior to name lookup.
-      domain = URI::Punycode.to_ascii domain
-
-      hints = LibC::Addrinfo.new
-      hints.ai_family = (family || Family::UNSPEC).to_i32
-      hints.ai_socktype = type
-      hints.ai_protocol = protocol
-      hints.ai_flags = 0
-
-      if service.is_a?(Int)
-        hints.ai_flags |= LibC::AI_NUMERICSERV
-      end
-
-      # On OS X < 10.12, the libsystem implementation of getaddrinfo segfaults
-      # if AI_NUMERICSERV is set, and servname is NULL or 0.
-      {% if flag?(:darwin) %}
-        if (service == 0 || service == nil) && (hints.ai_flags & LibC::AI_NUMERICSERV)
+      {% if flag?(:wasm32) %}
+        raise NotImplementedError.new "Socket::Addrinfo.getaddrinfo"
+      {% else %}
+        # RFC 3986 says:
+        # > When a non-ASCII registered name represents an internationalized domain name
+        # > intended for resolution via the DNS, the name must be transformed to the IDNA
+        # > encoding [RFC3490] prior to name lookup.
+        domain = URI::Punycode.to_ascii domain
+
+        hints = LibC::Addrinfo.new
+        hints.ai_family = (family || Family::UNSPEC).to_i32
+        hints.ai_socktype = type
+        hints.ai_protocol = protocol
+        hints.ai_flags = 0
+
+        if service.is_a?(Int)
           hints.ai_flags |= LibC::AI_NUMERICSERV
-          service = "00"
-        end
-      {% end %}
-      {% if flag?(:win32) %}
-        if service.is_a?(Int) && service < 0
-          raise Error.from_os_error(nil, WinError::WSATYPE_NOT_FOUND, domain: domain, type: type, protocol: protocol, service: service)
         end
-      {% end %}
 
-      ret = LibC.getaddrinfo(domain, service.to_s, pointerof(hints), out ptr)
-      unless ret.zero?
-        {% if flag?(:unix) %}
-          # EAI_SYSTEM is not defined on win32
-          if ret == LibC::EAI_SYSTEM
-            raise Error.from_os_error nil, Errno.value, domain: domain
+        # On OS X < 10.12, the libsystem implementation of getaddrinfo segfaults
+        # if AI_NUMERICSERV is set, and servname is NULL or 0.
+        {% if flag?(:darwin) %}
+          if (service == 0 || service == nil) && (hints.ai_flags & LibC::AI_NUMERICSERV)
+            hints.ai_flags |= LibC::AI_NUMERICSERV
+            service = "00"
+          end
+        {% end %}
+        {% if flag?(:win32) %}
+          if service.is_a?(Int) && service < 0
+            raise Error.from_os_error(nil, WinError::WSATYPE_NOT_FOUND, domain: domain, type: type, protocol: protocol, service: service)
           end
         {% end %}
 
-        error = {% if flag?(:win32) %}
-                  WinError.new(ret.to_u32!)
-                {% else %}
-                  Errno.new(ret)
-                {% end %}
-        raise Error.from_os_error(nil, error, domain: domain, type: type, protocol: protocol, service: service)
-      end
+        ret = LibC.getaddrinfo(domain, service.to_s, pointerof(hints), out ptr)
+        unless ret.zero?
+          {% if flag?(:unix) %}
+            # EAI_SYSTEM is not defined on win32
+            if ret == LibC::EAI_SYSTEM
+              raise Error.from_os_error nil, Errno.value, domain: domain
+            end
+          {% end %}
 
-      begin
-        yield new(ptr)
-      ensure
-        LibC.freeaddrinfo(ptr)
-      end
+          error = {% if flag?(:win32) %}
+                    WinError.new(ret.to_u32!)
+                  {% else %}
+                    Errno.new(ret)
+                  {% end %}
+          raise Error.from_os_error(nil, error, domain: domain, type: type, protocol: protocol, service: service)
+        end
+
+        begin
+          yield new(ptr)
+        ensure
+          LibC.freeaddrinfo(ptr)
+        end
+      {% end %}
     end
 
     # Resolves *domain* for the TCP protocol and returns an `Array` of possible
diff --git a/src/socket/common.cr b/src/socket/common.cr
index 2b55eb715992..19f1700a2cbd 100644
--- a/src/socket/common.cr
+++ b/src/socket/common.cr
@@ -1,6 +1,9 @@
 {% if flag?(:win32) %}
   require "c/ws2tcpip"
   require "c/afunix"
+{% elsif flag?(:wasi) %}
+  require "c/arpa/inet"
+  require "c/netinet/in"
 {% else %}
   require "c/arpa/inet"
   require "c/sys/un"
@@ -38,10 +41,12 @@ class Socket < IO
   end
 
   enum Type
-    STREAM    = LibC::SOCK_STREAM
-    DGRAM     = LibC::SOCK_DGRAM
-    RAW       = LibC::SOCK_RAW
-    SEQPACKET = LibC::SOCK_SEQPACKET
+    STREAM = LibC::SOCK_STREAM
+    DGRAM  = LibC::SOCK_DGRAM
+    {% unless flag?(:wasi) %}
+      RAW       = LibC::SOCK_RAW
+      SEQPACKET = LibC::SOCK_SEQPACKET
+    {% end %}
   end
 
   class Error < IO::Error
diff --git a/src/socket/unix_socket.cr b/src/socket/unix_socket.cr
index 9ec3a8188ef6..4efbd1c1c4bd 100644
--- a/src/socket/unix_socket.cr
+++ b/src/socket/unix_socket.cr
@@ -63,18 +63,22 @@ class UNIXSocket < Socket
   # left.gets # => "message"
   # ```
   def self.pair(type : Type = Type::STREAM) : {UNIXSocket, UNIXSocket}
-    fds = uninitialized Int32[2]
+    {% if flag?(:wasm32) %}
+      raise NotImplementedError.new "UNIXSocket.pair"
+    {% else %}
+      fds = uninitialized Int32[2]
 
-    socktype = type.value
-    {% if LibC.has_constant?(:SOCK_CLOEXEC) %}
+      socktype = type.value
+      {% if LibC.has_constant?(:SOCK_CLOEXEC) %}
       socktype |= LibC::SOCK_CLOEXEC
-    {% end %}
+      {% end %}
 
-    if LibC.socketpair(Family::UNIX, socktype, 0, fds) != 0
-      raise Socket::Error.new("socketpair:")
-    end
+      if LibC.socketpair(Family::UNIX, socktype, 0, fds) != 0
+        raise Socket::Error.new("socketpair:")
+      end
 
-    {UNIXSocket.new(fd: fds[0], type: type), UNIXSocket.new(fd: fds[1], type: type)}
+      {UNIXSocket.new(fd: fds[0], type: type), UNIXSocket.new(fd: fds[1], type: type)}
+    {% end %}
   end
 
   # Creates a pair of unnamed UNIX sockets (see `pair`) and yields them to the
diff --git a/src/spec.cr b/src/spec.cr
index 517acc720a58..5d370e71811a 100644
--- a/src/spec.cr
+++ b/src/spec.cr
@@ -127,7 +127,7 @@ end
 
 Spec.add_split_filter ENV["SPEC_SPLIT"]?
 
-{% unless flag?(:win32) %}
+{% unless flag?(:win32) || flag?(:wasm32) %}
   # TODO(windows): re-enable this once Signal is ported
   Signal::INT.trap { Spec.abort! }
 {% end %}
diff --git a/src/system_error.cr b/src/system_error.cr
index f7e4653cc27a..7d434695b003 100644
--- a/src/system_error.cr
+++ b/src/system_error.cr
@@ -43,7 +43,7 @@ module SystemError
   end
 
   # The original system error wrapped by this exception
-  getter os_error : Errno | WinError | Nil
+  getter os_error : Errno | WinError | WasiError | Nil
 
   # :nodoc:
   protected def os_error=(@os_error)
@@ -54,7 +54,7 @@ module SystemError
     #
     # The system message corresponding to the OS error value amends the *message*.
     # Additional keyword arguments are forwarded to the exception initializer `.new_from_os_error`.
-    def from_os_error(message : String?, os_error : Errno | WinError | Nil, **opts)
+    def from_os_error(message : String?, os_error : Errno | WinError | WasiError | Nil, **opts)
       message = self.build_message(message, **opts)
       message =
         if message
@@ -94,7 +94,7 @@ module SystemError
     # By default it returns the result of `Errno#message` or `WinError#message`.
     # This method can be overridden for customization of the error message based
     # on *or_error*  and *\*\*opts*.
-    protected def os_error_message(os_error : Errno | WinError | Nil, **opts) : String?
+    protected def os_error_message(os_error : Errno | WinError | WasiError | Nil, **opts) : String?
       os_error.try &.message
     end
 
diff --git a/src/wasi_error.cr b/src/wasi_error.cr
new file mode 100644
index 000000000000..9a04ed315463
--- /dev/null
+++ b/src/wasi_error.cr
@@ -0,0 +1,249 @@
+enum WasiError : UInt16
+  # Returns the system error message associated with this error code.
+  def message : String
+    case self
+    when SUCCESS        then "No error occurred. System call completed successfully."
+    when TOOBIG         then "Argument list too long."
+    when ACCES          then "Permission denied."
+    when ADDRINUSE      then "Address in use."
+    when ADDRNOTAVAIL   then "Address not available."
+    when AFNOSUPPORT    then "Address family not supported."
+    when AGAIN          then "Resource unavailable, or operation would block."
+    when ALREADY        then "Connection already in progress."
+    when BADF           then "Bad file descriptor."
+    when BADMSG         then "Bad message."
+    when BUSY           then "Device or resource busy."
+    when CANCELED       then "Operation canceled."
+    when CHILD          then "No child processes."
+    when CONNABORTED    then "Connection aborted."
+    when CONNREFUSED    then "Connection refused."
+    when CONNRESET      then "Connection reset."
+    when DEADLK         then "Resource deadlock would occur."
+    when DESTADDRREQ    then "Destination address required."
+    when DOM            then "Mathematics argument out of domain of function."
+    when DQUOT          then "Reserved."
+    when EXIST          then "File exists."
+    when FAULT          then "Bad address."
+    when FBIG           then "File too large."
+    when HOSTUNREACH    then "Host is unreachable."
+    when IDRM           then "Identifier removed."
+    when ILSEQ          then "Illegal byte sequence."
+    when INPROGRESS     then "Operation in progress."
+    when INTR           then "Interrupted function."
+    when INVAL          then "Invalid argument."
+    when IO             then "I/O error."
+    when ISCONN         then "Socket is connected."
+    when ISDIR          then "Is a directory."
+    when LOOP           then "Too many levels of symbolic links."
+    when MFILE          then "File descriptor value too large."
+    when MLINK          then "Too many links."
+    when MSGSIZE        then "Message too large."
+    when MULTIHOP       then "Reserved."
+    when NAMETOOLONG    then "Filename too long."
+    when NETDOWN        then "Network is down."
+    when NETRESET       then "Connection aborted by network."
+    when NETUNREACH     then "Network unreachable."
+    when NFILE          then "Too many files open in system."
+    when NOBUFS         then "No buffer space available."
+    when NODEV          then "No such device."
+    when NOENT          then "No such file or directory."
+    when NOEXEC         then "Executable file format error."
+    when NOLCK          then "No locks available."
+    when NOLINK         then "Reserved."
+    when NOMEM          then "Not enough space."
+    when NOMSG          then "No message of the desired type."
+    when NOPROTOOPT     then "Protocol not available."
+    when NOSPC          then "No space left on device."
+    when NOSYS          then "Function not supported."
+    when NOTCONN        then "The socket is not connected."
+    when NOTDIR         then "Not a directory or a symbolic link to a directory."
+    when NOTEMPTY       then "Directory not empty."
+    when NOTRECOVERABLE then "State not recoverable."
+    when NOTSOCK        then "Not a socket."
+    when NOTSUP         then "Not supported, or operation not supported on socket."
+    when NOTTY          then "Inappropriate I/O control operation."
+    when NXIO           then "No such device or address."
+    when OVERFLOW       then "Value too large to be stored in data type."
+    when OWNERDEAD      then "Previous owner died."
+    when PERM           then "Operation not permitted."
+    when PIPE           then "Broken pipe."
+    when PROTO          then "Protocol error."
+    when PROTONOSUPPORT then "Protocol not supported."
+    when PROTOTYPE      then "Protocol wrong type for socket."
+    when RANGE          then "Result too large."
+    when ROFS           then "Read-only file system."
+    when SPIPE          then "Invalid seek."
+    when SRCH           then "No such process."
+    when STALE          then "Reserved."
+    when TIMEDOUT       then "Connection timed out."
+    when TXTBSY         then "Text file busy."
+    when XDEV           then "Cross-device link."
+    when NOTCAPABLE     then "Extension: Capabilities insufficient."
+    else                     "Unknown error."
+    end
+  end
+
+  # Transforms this `WasiError` value to the equivalent `Errno` value.
+  #
+  # This is only defined for some values. If no transformation is defined for
+  # a specific value, the default result is `Errno::EINVAL`.
+  def to_errno : Errno
+    case self
+    when TOOBIG         then Errno::E2BIG
+    when ACCES          then Errno::EACCES
+    when ADDRINUSE      then Errno::EADDRINUSE
+    when ADDRNOTAVAIL   then Errno::EADDRNOTAVAIL
+    when AFNOSUPPORT    then Errno::EAFNOSUPPORT
+    when AGAIN          then Errno::EAGAIN
+    when ALREADY        then Errno::EALREADY
+    when BADF           then Errno::EBADF
+    when BADMSG         then Errno::EBADMSG
+    when BUSY           then Errno::EBUSY
+    when CANCELED       then Errno::ECANCELED
+    when CHILD          then Errno::ECHILD
+    when CONNABORTED    then Errno::ECONNABORTED
+    when CONNREFUSED    then Errno::ECONNREFUSED
+    when CONNRESET      then Errno::ECONNRESET
+    when DEADLK         then Errno::EDEADLK
+    when DESTADDRREQ    then Errno::EDESTADDRREQ
+    when DOM            then Errno::EDOM
+    when DQUOT          then Errno::EDQUOT
+    when EXIST          then Errno::EEXIST
+    when FAULT          then Errno::EFAULT
+    when FBIG           then Errno::EFBIG
+    when HOSTUNREACH    then Errno::EHOSTUNREACH
+    when IDRM           then Errno::EIDRM
+    when ILSEQ          then Errno::EILSEQ
+    when INPROGRESS     then Errno::EINPROGRESS
+    when INTR           then Errno::EINTR
+    when INVAL          then Errno::EINVAL
+    when IO             then Errno::EIO
+    when ISCONN         then Errno::EISCONN
+    when ISDIR          then Errno::EISDIR
+    when LOOP           then Errno::ELOOP
+    when MFILE          then Errno::EMFILE
+    when MLINK          then Errno::EMLINK
+    when MSGSIZE        then Errno::EMSGSIZE
+    when MULTIHOP       then Errno::EMULTIHOP
+    when NAMETOOLONG    then Errno::ENAMETOOLONG
+    when NETDOWN        then Errno::ENETDOWN
+    when NETRESET       then Errno::ENETRESET
+    when NETUNREACH     then Errno::ENETUNREACH
+    when NFILE          then Errno::ENFILE
+    when NOBUFS         then Errno::ENOBUFS
+    when NODEV          then Errno::ENODEV
+    when NOENT          then Errno::ENOENT
+    when NOEXEC         then Errno::ENOEXEC
+    when NOLCK          then Errno::ENOLCK
+    when NOLINK         then Errno::ENOLINK
+    when NOMEM          then Errno::ENOMEM
+    when NOMSG          then Errno::ENOMSG
+    when NOPROTOOPT     then Errno::ENOPROTOOPT
+    when NOSPC          then Errno::ENOSPC
+    when NOSYS          then Errno::ENOSYS
+    when NOTCONN        then Errno::ENOTCONN
+    when NOTDIR         then Errno::ENOTDIR
+    when NOTEMPTY       then Errno::ENOTEMPTY
+    when NOTRECOVERABLE then Errno::ENOTRECOVERABLE
+    when NOTSOCK        then Errno::ENOTSOCK
+    when NOTSUP         then Errno::ENOTSUP
+    when NOTTY          then Errno::ENOTTY
+    when NXIO           then Errno::ENXIO
+    when OVERFLOW       then Errno::EOVERFLOW
+    when OWNERDEAD      then Errno::EOWNERDEAD
+    when PERM           then Errno::EPERM
+    when PIPE           then Errno::EPIPE
+    when PROTO          then Errno::EPROTO
+    when PROTONOSUPPORT then Errno::EPROTONOSUPPORT
+    when PROTOTYPE      then Errno::EPROTOTYPE
+    when RANGE          then Errno::ERANGE
+    when ROFS           then Errno::EROFS
+    when SPIPE          then Errno::ESPIPE
+    when SRCH           then Errno::ESRCH
+    when STALE          then Errno::ESTALE
+    when TIMEDOUT       then Errno::ETIMEDOUT
+    when TXTBSY         then Errno::ETXTBSY
+    when XDEV           then Errno::EXDEV
+    when NOTCAPABLE     then Errno::ENOTCAPABLE
+    else                     Errno::EINVAL
+    end
+  end
+
+  SUCCESS        # No error occurred. System call completed successfully.
+  TOOBIG         # Argument list too long.
+  ACCES          # Permission denied.
+  ADDRINUSE      # Address in use.
+  ADDRNOTAVAIL   # Address not available.
+  AFNOSUPPORT    # Address family not supported.
+  AGAIN          # Resource unavailable, or operation would block.
+  ALREADY        # Connection already in progress.
+  BADF           # Bad file descriptor.
+  BADMSG         # Bad message.
+  BUSY           # Device or resource busy.
+  CANCELED       # Operation canceled.
+  CHILD          # No child processes.
+  CONNABORTED    # Connection aborted.
+  CONNREFUSED    # Connection refused.
+  CONNRESET      # Connection reset.
+  DEADLK         # Resource deadlock would occur.
+  DESTADDRREQ    # Destination address required.
+  DOM            # Mathematics argument out of domain of function.
+  DQUOT          # Reserved.
+  EXIST          # File exists.
+  FAULT          # Bad address.
+  FBIG           # File too large.
+  HOSTUNREACH    # Host is unreachable.
+  IDRM           # Identifier removed.
+  ILSEQ          # Illegal byte sequence.
+  INPROGRESS     # Operation in progress.
+  INTR           # Interrupted function.
+  INVAL          # Invalid argument.
+  IO             # I/O error.
+  ISCONN         # Socket is connected.
+  ISDIR          # Is a directory.
+  LOOP           # Too many levels of symbolic links.
+  MFILE          # File descriptor value too large.
+  MLINK          # Too many links.
+  MSGSIZE        # Message too large.
+  MULTIHOP       # Reserved.
+  NAMETOOLONG    # Filename too long.
+  NETDOWN        # Network is down.
+  NETRESET       # Connection aborted by network.
+  NETUNREACH     # Network unreachable.
+  NFILE          # Too many files open in system.
+  NOBUFS         # No buffer space available.
+  NODEV          # No such device.
+  NOENT          # No such file or directory.
+  NOEXEC         # Executable file format error.
+  NOLCK          # No locks available.
+  NOLINK         # Reserved.
+  NOMEM          # Not enough space.
+  NOMSG          # No message of the desired type.
+  NOPROTOOPT     # Protocol not available.
+  NOSPC          # No space left on device.
+  NOSYS          # Function not supported.
+  NOTCONN        # The socket is not connected.
+  NOTDIR         # Not a directory or a symbolic link to a directory.
+  NOTEMPTY       # Directory not empty.
+  NOTRECOVERABLE # State not recoverable.
+  NOTSOCK        # Not a socket.
+  NOTSUP         # Not supported, or operation not supported on socket.
+  NOTTY          # Inappropriate I/O control operation.
+  NXIO           # No such device or address.
+  OVERFLOW       # Value too large to be stored in data type.
+  OWNERDEAD      # Previous owner died.
+  PERM           # Operation not permitted.
+  PIPE           # Broken pipe.
+  PROTO          # Protocol error.
+  PROTONOSUPPORT # Protocol not supported.
+  PROTOTYPE      # Protocol wrong type for socket.
+  RANGE          # Result too large.
+  ROFS           # Read-only file system.
+  SPIPE          # Invalid seek.
+  SRCH           # No such process.
+  STALE          # Reserved.
+  TIMEDOUT       # Connection timed out.
+  TXTBSY         # Text file busy.
+  XDEV           # Cross-device link.
+  NOTCAPABLE     # Extension: Capabilities insufficient.
+end