diff --git a/README.md b/README.md
index e3f9a7c..6835bab 100644
--- a/README.md
+++ b/README.md
@@ -43,7 +43,7 @@ Let's make an empty file named **Seedfile** in the directory where your Xcode pr
 github "Alamofire/Alamofire", "1.2.1", :files => "Source/*.{swift,h}"
 github "devxoul/JLToast", "1.2.5", :files => "JLToast/*.{swift,h}"
 github "devxoul/SwipeBack", "1.0.4"
-github "Masonry/SnapKit", "0.10.0", :files => "Source/*.{swift,h}"
+github "SnapKit/SnapKit", :commit => "62e7645", :files => "Source/*.{swift,h}"
 
 target :MyAppTest do
   github "Quick/Quick", "v0.3.1", :files => "Quick/**.{swift,h}"
@@ -58,14 +58,14 @@ Each line in a Seedfile consists of three parts: source, tag, and files. Let's l
 ```ruby
 github "devxoul/JLToast", "1.2.5", :files => "JLToast/*.{swift,h}"
 ~~~~~~~~~~~~~~~~~~~~~~~~  ~~~~~~~  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-       (Source)            (Tag)              (Files)
+       (Source)          (Version)             (Files)
 ```
 
-| Parts  | Example                           | Required | Default               |
-|--------|-----------------------------------|:--------:|:---------------------:|
-| Source | `github "devxoul/SwipeBack"`      | Required | -                     |
-| Tag    | `1.0.4`                           | Required | -                     |
-| Files  | `:files => "JLToast/*.{swift,h}"` | Optional | `*/**.{h,m,mm,swift}` |
+| Parts   | Example                           | Required | Default               |
+|---------|-----------------------------------|:--------:|:---------------------:|
+| Source  | `github "devxoul/SwipeBack"`      | Required | -                     |
+| Version | Tag: `"1.0.4"`<br>Branch: `"swift-2.0"`<br>Commit: `:commit => "SHA1"` | Required | -                     |
+| Files   | `:files => "JLToast/*.{swift,h}"` | Optional | `*/**.{h,m,mm,swift}` |
 
 > **Tip:** You can pass an array to `:files` for multiple file patterns:
 >
diff --git a/lib/cocoaseeds/core.rb b/lib/cocoaseeds/core.rb
index c5cceaa..f99e9c3 100644
--- a/lib/cocoaseeds/core.rb
+++ b/lib/cocoaseeds/core.rb
@@ -161,6 +161,10 @@ def analyze_dependencies
           seed = Seeds::Seed.new
           seed.name = lock.split(' (')[0]
           seed.version = lock.split('(')[1].split(')')[0]
+          if seed.version.start_with? '$'
+            seed.commit = seed.version[1..-1]
+            seed.version = nil
+          end
           self.locks[seed.name] = seed
         end
       end
@@ -224,8 +228,17 @@ def github(repo, tag, options={})
           seed = Seeds::Seed::GitHub.new
           seed.url = "https://github.com/#{repo}"
           seed.name = repo.split('/')[1]
-          seed.version = tag
-          seed.files = options[:files] || '**/*.{h,m,mm,swift}'
+          if tag.is_a?(String)
+            if options[:commit]
+              raise Seeds::Exception.new\
+                "#{repo}: Version and commit are both specified."
+            end
+            seed.version = tag
+            seed.files = options[:files] || '**/*.{h,m,mm,swift}'
+          elsif tag.is_a?(Hash)
+            seed.commit = tag[:commit][0..6]
+            seed.files = tag[:files] || '**/*.{h,m,mm,swift}'
+          end
           if seed.files.kind_of?(String)
             seed.files = [seed.files]
           end
@@ -256,8 +269,17 @@ def bitbucket(repo, tag, options={})
           seed = Seeds::Seed::BitBucket.new
           seed.url = "https://bitbucket.org/#{repo}"
           seed.name = repo.split('/')[1]
-          seed.version = tag
-          seed.files = options[:files] || '**/*.{h,m,mm,swift}'
+          if tag.is_a?(String)
+            if options[:commit]
+              raise Seeds::Exception.new\
+                "#{repo}: Version and commit are both specified."
+            end
+            seed.version = tag
+            seed.files = options[:files] || '**/*.{h,m,mm,swift}'
+          elsif tag.is_a?(Hash)
+            seed.commit = tag[:commit][0..6]
+            seed.files = tag[:files] || '**/*.{h,m,mm,swift}'
+          end
           if seed.files.kind_of?(String)
             seed.files = [seed.files]
           end
@@ -289,53 +311,97 @@ def remove_seeds
     #
     def install_seeds
       self.seeds.sort.each do |name, seed|
-        dirname = File.join(self.root_path, "Seeds", name)
+        dirname = File.join(self.root_path, "Seeds", seed.name)
+        self.install_seed(seed, dirname)
+
+        next if not seed.files
+
+        # add seed files to `source_files`
+        self.source_files[name] = []
+        seed.files.each do |file|
+          paths = Dir.glob(File.join(dirname, file))
+          paths.each do |path|
+            path = self.path_with_prefix(seed.name, path)
+            self.source_files[name].push(path)
+          end
+        end
+      end
+    end
 
-        if lock = self.locks[name]
-          old_version = lock.version
+
+    # Installs new seed or updates existing seed in {#dirname}.
+    #
+    # @!visibility private
+    #
+    def install_seed(seed, dirname)
+      # clone and return if not exists
+      if not File.exist?(dirname)
+        say "Installing #{seed.name} (#{seed.version or seed.commit})".green
+
+        command = "git clone #{seed.url}"
+        command += " -b #{seed.version}" if seed.version
+        command += " #{dirname} 2>&1"
+        output = `#{command}`
+
+        not_found = output.include?("not found")
+        if not_found and output.include?("repository")
+          say "[!] #{seed.name}: Couldn't find the repository.".red
+        elsif not_found and output.include?("upstream")
+          say "[!] #{seed.name}: Couldn't find the tag `#{seed.version}`.".red
         end
 
-        if File.exist?(dirname)
-          tag = `cd #{dirname} && git describe --tags --abbrev=0 2>&1`
-          tag.strip!
-          if tag == seed.version and (not old_version or tag == old_version)
-            say "Using #{name} (#{seed.version})"
-            `cd #{dirname} 2>&1 &&\
-             git reset HEAD --hard 2>&1 &&\
-             git checkout . 2>&1 &&\
-             git clean -fd 2>&1`
-          else
-            say "Installing #{name} #{seed.version} (was #{old_version})".green
-            `cd #{dirname} 2>&1 &&\
-             git reset HEAD --hard 2>&1 &&\
-             git checkout . 2>&1 &&\
-             git clean -fd 2>&1 &&\
-             git fetch origin #{seed.version} --tags 2>&1 &&\
-             git checkout #{seed.version} 2>&1`
-          end
-        else
-          say "Installing #{name} (#{seed.version})".green
-          output = `git clone #{seed.url} -b #{seed.version} #{dirname} 2>&1`
-          if output.include?("not found")
-            if output.include?("repository")
-              say "[!] #{name}: Couldn't find the repository.".red
-            elsif output.include?("upstream")
-              say "[!] #{name}: Couldn't find the tag `#{seed.version}`.".red
-            end
+        if seed.commit and not seed.version # checkout to commit
+          output = `cd #{dirname} 2>&1 && git checkout #{seed.commit} 2>&1`
+          if output.include?("did not match any")
+            say "[!] #{seed.name}: Couldn't find the commit "\
+                "`#{seed.commit}`.".red
           end
         end
 
-        if seed.files
-          self.source_files[name] = []
-          seed.files.each do |file|
-            paths = Dir.glob(File.join(dirname, file))
-            paths.each do |path|
-              path = self.path_with_prefix(seed.name, path)
-              self.source_files[name].push(path)
-            end
-          end
+        return
+      end
+
+      # discard local changes
+      `cd #{dirname} 2>&1 &&\
+       git reset HEAD --hard 2>&1 &&\
+       git checkout . 2>&1 &&\
+       git clean -fd 2>&1`
+
+      if lock = self.locks[seed.name]
+        lock_version = lock.version
+        lock_commit = lock.commit
+      end
+
+      if seed.version == lock_version and seed.commit == lock_commit
+        say "Using #{seed.name} (#{lock_version or lock_commit})"
+        return
+      end
+
+      if seed.version
+        say "Installing #{seed.name} #{seed.version}"\
+            " (was #{lock_version or lock_commit})".green
+        output = `cd #{dirname} 2>&1 &&\
+                 git fetch origin #{seed.version} --tags 2>&1 &&\
+                 git checkout #{seed.version} 2>&1`
+        puts output
+        if output.include?("Couldn't find")
+          say "[!] #{seed.name}: Couldn't find the tag or branch named "\
+              "`#{seed.commit}`.".red
+        end
+
+      elsif seed.commit
+        say "Installing #{seed.name} #{seed.commit}"\
+            " (was #{lock_version or lock_commit})".green
+        output = `cd #{dirname} 2>&1 &&
+                  git checkout master 2>&1 &&
+                  git pull 2>&1 &&
+                  git checkout #{seed.commit} 2>&1`
+        if output.include?("did not match any")
+          say "[!] #{seed.name}: Couldn't find the commit "\
+              "`#{seed.commit}`.".red
         end
       end
+
     end
 
     # Append seed name as a prefix to file name and returns the path.
@@ -463,7 +529,7 @@ def configure_phase
     def build_lockfile
       tree = { "SEEDS" => [] }
       self.seeds.each do |name, seed|
-        tree["SEEDS"] << "#{name} (#{seed.version})"
+        tree["SEEDS"] << "#{name} (#{seed.version or '$' + seed.commit})"
       end
       File.write(self.lockfile_path, YAML.dump(tree))
     end
diff --git a/lib/cocoaseeds/seed.rb b/lib/cocoaseeds/seed.rb
index 5a7eb0d..9c8f47c 100644
--- a/lib/cocoaseeds/seed.rb
+++ b/lib/cocoaseeds/seed.rb
@@ -9,6 +9,10 @@ class Seed
     #
     attr_accessor :version
 
+    # @return [String] the commit hash of the seed
+    #
+    attr_accessor :commit
+
     # @return [String] the url of the seed
     #
     attr_accessor :url
diff --git a/test/test_install.rb b/test/test_install.rb
index ed8f820..dee43e3 100644
--- a/test/test_install.rb
+++ b/test/test_install.rb
@@ -2,7 +2,7 @@
 
 class InstallTest < Test
 
-  def test_install_github
+  def test_github_version
     seedfile %{
       github "devxoul/JLToast", "1.2.2", :files => "JLToast/*.{h,swift}"
     }
@@ -14,6 +14,68 @@ def test_install_github
   end
 
 
+  def test_github_commit_1
+    seedfile %{
+      github "devxoul/SwipeBack", :commit => "534a677"
+    }
+    @seed.install
+
+    dir = File.join(@seeds_dirname, "SwipeBack")
+
+    assert\
+      File.exists?(dir),
+      "Directory Seeds/SwipeBack not exists."
+
+    refute\
+      File.exists?(File.join(dir, "SwipeBack/SwipeBack.h")),
+      "File Seeds/SwipeBack/SwipeBack/SwipeBack.h should not exist."
+  end
+
+
+  def test_github_commit_2
+    seedfile %{
+      github "devxoul/SwipeBack", :commit => "90b256"
+    }
+    @seed.install
+
+    dir = File.join(@seeds_dirname, "SwipeBack")
+
+    assert\
+      File.exists?(dir),
+      "Directory Seeds/SwipeBack not exists."
+
+    assert\
+      File.exists?(File.join(dir, "SwipeBack/SwipeBack.h")),
+      "File Seeds/SwipeBack/SwipeBack/SwipeBack.h does not exist."
+  end
+
+
+  def test_github_update
+    path = File.join(@seeds_dirname, "SwiftyImage/SwiftyImage/SwiftyImage.h")
+
+    seedfile %{
+      github "devxoul/SwiftyImage", :commit => "af84dbd"
+    }
+    @seed.install
+    refute File.exists?(path),
+      "File Seeds/SwiftyImage/SwiftyImage/SwiftyImage.h should not exist."
+
+    seedfile %{
+      github "devxoul/SwiftyImage", :commit => "8b0c07a"
+    }
+    @seed.install
+    assert File.exists?(path),
+      "File Seeds/SwiftyImage/SwiftyImage/SwiftyImage.h does not exist."
+
+    seedfile %{
+      github "devxoul/SwiftyImage", :commit => "af84dbd"
+    }
+    @seed.install
+    refute File.exists?(path),
+      "File Seeds/SwiftyImage/SwiftyImage/SwiftyImage.h should not exist."
+  end
+
+
   def test_raise_invalid_github_reponame
     seedfile %{
       github "JLToast", "1.2.2"
@@ -21,6 +83,14 @@ def test_raise_invalid_github_reponame
     assert_raises Seeds::Exception do @seed.install end
   end
 
+
+  def test_raise_github_both_tag_and_commit
+    seedfile %{
+      github "JLToast", "1.2.2", :commit => "83a1b50"
+    }
+    assert_raises Seeds::Exception do @seed.install end
+  end
+
 end
 
 
@@ -45,4 +115,12 @@ def test_raise_invalid_bitbucket_reponame
     assert_raises Seeds::Exception do @seed.install end
   end
 
+
+  def test_raise_bitbucket_both_tag_and_commit
+    seedfile %{
+      bitbucket "JLToast", "1.2.2", :commit => "83a1b50"
+    }
+    assert_raises Seeds::Exception do @seed.install end
+  end
+
 end
diff --git a/test/test_seed.rb b/test/test_seed.rb
index e7648d1..262fa09 100644
--- a/test/test_seed.rb
+++ b/test/test_seed.rb
@@ -15,6 +15,20 @@ def test_github_version
   end
 
 
+  def test_github_commit
+    seedfile %{
+      github "devxoul/JLToast", :commit => "83a1b50153ed26f0ae0e90d65"
+    }
+    @seed.prepare_requirements
+    @seed.execute_seedfile
+    s = @seed.seeds["JLToast"]
+    assert_equal s.name, "JLToast"
+    assert_nil s.version
+    assert_equal s.commit, "83a1b50"
+    assert_equal s.url, "https://github.com/devxoul/JLToast"
+  end
+
+
   def test_github_version_and_files
     seedfile %{
       github "devxoul/JLToast", "1.3.2", :files => "JLToast/*.{h,swift}"
@@ -44,6 +58,27 @@ def test_github_version_and_files_array
   end
 
 
+  def test_github_commit_and_files
+    seedfile %{
+      github "devxoul/JLToast",
+        :commit => "83a1b50",
+        :files => "JLToast/*.{h,swift}"
+    }
+    @seed.prepare_requirements
+    @seed.execute_seedfile
+    s = @seed.seeds["JLToast"]
+    assert_equal s.name, "JLToast"
+    assert_nil s.version
+    assert_equal s.commit, "83a1b50"
+    assert_equal s.url, "https://github.com/devxoul/JLToast"
+    assert_equal s.files, ["JLToast/*.{h,swift}"]
+  end
+
+end
+
+
+class SeedTest
+
   def test_bitbucket_version
     seedfile %{
       bitbucket "devxoul/JLToast", "1.3.2"
@@ -57,6 +92,20 @@ def test_bitbucket_version
   end
 
 
+  def test_bitbucket_commit
+    seedfile %{
+      github "devxoul/JLToast", :commit => "83a1b50"
+    }
+    @seed.prepare_requirements
+    @seed.execute_seedfile
+    s = @seed.seeds["JLToast"]
+    assert_equal s.name, "JLToast"
+    assert_nil s.version
+    assert_equal s.commit, "83a1b50"
+    assert_equal s.url, "https://github.com/devxoul/JLToast"
+  end
+
+
   def test_bitbucket_version_and_files
     seedfile %{
       bitbucket "devxoul/JLToast", "1.3.2", :files => "JLToast/*.{h,swift}"
@@ -85,4 +134,21 @@ def test_bitbucket_version_and_files_array
     assert_equal s.files, ["JLToast/JLToast.h", "JLToast/JLToast.swift"]
   end
 
+
+  def test_bitbucket_commit_and_files
+    seedfile %{
+      github "devxoul/JLToast",
+        :commit => "83a1b50",
+        :files => "JLToast/*.{h,swift}"
+    }
+    @seed.prepare_requirements
+    @seed.execute_seedfile
+    s = @seed.seeds["JLToast"]
+    assert_equal s.name, "JLToast"
+    assert_nil s.version
+    assert_equal s.commit, "83a1b50"
+    assert_equal s.url, "https://github.com/devxoul/JLToast"
+    assert_equal s.files, ["JLToast/*.{h,swift}"]
+  end
+
 end