diff --git a/noxfile.py b/noxfile.py
index 7b1fd9b..4b15f4d 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -14,246 +14,260 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-# Generated by synthtool. DO NOT EDIT!
-
-from __future__ import absolute_import
 import os
 import pathlib
+from pathlib import Path
+import re
 import shutil
-import warnings
 
 import nox
 
+
 BLACK_VERSION = "black==22.3.0"
-ISORT_VERSION = "isort==5.10.1"
-LINT_PATHS = ["docs", "google", "tests", "noxfile.py", "setup.py"]
+LINT_PATHS = ["docs", "google", "noxfile.py", "setup.py"]
 
-# NOTE: Pin the version of grpcio-tools to 1.48.2 for compatibility with
-# Protobuf 3.19.5. Please ensure that the minimum required version of
-# protobuf in setup.py is compatible with the pb2 files generated
-# by grpcio-tools before changing the pinned version below.
-GRPCIO_TOOLS_VERSION = "grpcio-tools==1.48.2"
 
-DEFAULT_PYTHON_VERSION = "3.8"
-
-UNIT_TEST_PYTHON_VERSIONS = ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
-UNIT_TEST_STANDARD_DEPENDENCIES = [
-    "mock",
-    "asyncmock",
-    "pytest",
-    "pytest-cov",
-    "pytest-asyncio",
-]
-UNIT_TEST_EXTERNAL_DEPENDENCIES = []
-UNIT_TEST_LOCAL_DEPENDENCIES = []
-UNIT_TEST_DEPENDENCIES = []
-UNIT_TEST_EXTRAS = []
-UNIT_TEST_EXTRAS_BY_PYTHON = {}
-
-SYSTEM_TEST_PYTHON_VERSIONS = ["3.8"]
-SYSTEM_TEST_STANDARD_DEPENDENCIES = [
-    "mock",
-    "pytest",
-    "google-cloud-testutils",
-]
-SYSTEM_TEST_EXTERNAL_DEPENDENCIES = []
-SYSTEM_TEST_LOCAL_DEPENDENCIES = []
-SYSTEM_TEST_DEPENDENCIES = []
-SYSTEM_TEST_EXTRAS = []
-SYSTEM_TEST_EXTRAS_BY_PYTHON = {}
+# `grpcio-tools` 1.59.0 or newer is required for protobuf 5.x compatibility.
+GRPCIO_TOOLS_VERSION = "grpcio-tools==1.59.0"
 
 CURRENT_DIRECTORY = pathlib.Path(__file__).parent.absolute()
-
-# 'docfx' is excluded since it only needs to run in 'docs-presubmit'
-nox.options.sessions = [
-    "system",
-    "lint",
-    "integration_test",
-    "lint_setup_py",
-    "blacken",
-    "docs",
-    "unit",
-]
-
-# Error if a python version is missing
-nox.options.error_on_missing_interpreters = True
-
-
-@nox.session(python=DEFAULT_PYTHON_VERSION)
-def lint(session):
-    """Run linters.
-
-    Returns a failure if the linters find linting errors or sufficiently
-    serious code quality issues.
-    """
-    session.install("flake8", BLACK_VERSION)
-    session.run(
-        "black",
-        "--check",
-        *LINT_PATHS,
-    )
-    session.run("flake8", "google", "tests")
+UNIT_TEST_PYTHON_VERSIONS = ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
 
 
-@nox.session(python=DEFAULT_PYTHON_VERSION)
+@nox.session(python="3.8")
 def blacken(session):
-    """Run black. Format code to uniform standard."""
-    session.install(BLACK_VERSION)
-    session.run(
-        "black",
-        *LINT_PATHS,
-    )
-
-
-@nox.session(python=DEFAULT_PYTHON_VERSION)
-def format(session):
-    """
-    Run isort to sort imports. Then run black
-    to format code to uniform standard.
+    """Run black.
+    Format code to uniform standard.
+    This currently uses Python 3.6 due to the automated Kokoro run of synthtool.
+    That run uses an image that doesn't have 3.6 installed. Before updating this
+    check the state of the `gcp_ubuntu_config` we use for that Kokoro run.
     """
-    session.install(BLACK_VERSION, ISORT_VERSION)
-    # Use the --fss option to sort imports using strict alphabetical order.
-    # See https://pycqa.github.io/isort/docs/configuration/options.html#force-sort-within-sections
-    session.run(
-        "isort",
-        "--fss",
-        *LINT_PATHS,
-    )
-    session.run(
-        "black",
-        *LINT_PATHS,
-    )
+    session.install(BLACK_VERSION)
+    session.run("black", "google", "setup.py")
 
 
-@nox.session(python=DEFAULT_PYTHON_VERSION)
+@nox.session(python="3.8")
 def lint_setup_py(session):
-    """Verify that setup.py is valid (including RST check)."""
+    """Verify that setup.py is valid"""
     session.install("docutils", "pygments")
-    session.run("python", "setup.py", "check", "--restructuredtext", "--strict")
+    session.run("python", "setup.py", "check", "--strict")
 
 
-def install_unittest_dependencies(session, *constraints):
-    standard_deps = UNIT_TEST_STANDARD_DEPENDENCIES + UNIT_TEST_DEPENDENCIES
-    session.install(*standard_deps, *constraints)
+def unit(session, repository, package, prerelease, protobuf_implementation):
+    """Run the unit test suite."""
+    downstream_dir = repository
+    if package:
+        downstream_dir = f"{repository}/packages/{package}"
 
-    if UNIT_TEST_EXTERNAL_DEPENDENCIES:
-        warnings.warn(
-            "'unit_test_external_dependencies' is deprecated. Instead, please "
-            "use 'unit_test_dependencies' or 'unit_test_local_dependencies'.",
-            DeprecationWarning,
-        )
-        session.install(*UNIT_TEST_EXTERNAL_DEPENDENCIES, *constraints)
+    # Install all test dependencies, then install this package in-place.
+    session.install("asyncmock", "pytest-asyncio")
 
-    if UNIT_TEST_LOCAL_DEPENDENCIES:
-        session.install(*UNIT_TEST_LOCAL_DEPENDENCIES, *constraints)
+    # Pin mock due to https://github.com/googleapis/python-pubsub/issues/840
+    session.install("mock==5.0.0", "pytest", "pytest-cov")
 
-    if UNIT_TEST_EXTRAS_BY_PYTHON:
-        extras = UNIT_TEST_EXTRAS_BY_PYTHON.get(session.python, [])
-    elif UNIT_TEST_EXTRAS:
-        extras = UNIT_TEST_EXTRAS
-    else:
-        extras = []
+    install_command = ["-e", f"{CURRENT_DIRECTORY}/{downstream_dir}"]
 
-    if extras:
-        session.install("-e", f".[{','.join(extras)}]", *constraints)
+    if prerelease:
+        install_prerelease_dependencies(
+            session,
+            f"{CURRENT_DIRECTORY}/{downstream_dir}/testing/constraints-{UNIT_TEST_PYTHON_VERSIONS[0]}.txt",
+        )
+        # Use the `--no-deps` options to allow pre-release versions of dependencies to be installed
+        install_command.extend(["--no-deps"])
     else:
-        session.install("-e", ".", *constraints)
+        # Install the pinned dependencies in constraints file
+        install_command.extend(
+            [
+                "-c",
+                f"{CURRENT_DIRECTORY}/{downstream_dir}/testing/constraints-{session.python}.txt",
+            ]
+        )
 
+    # These *must* be the last 3 install commands to get the packages from source.
+    session.install(*install_command)
 
-def default(session):
-    # Install all test dependencies, then install this package in-place.
+    # Remove the 'cpp' implementation once support for Protobuf 3.x is dropped.
+    # The 'cpp' implementation requires Protobuf<4.
+    if protobuf_implementation == "cpp":
+        session.install("protobuf<4")
 
-    constraints_path = str(
-        CURRENT_DIRECTORY / "testing" / f"constraints-{session.python}.txt"
-    )
-    install_unittest_dependencies(session, "-c", constraints_path)
+    # Install this library from source
+    session.install(".", "--no-deps")
 
-    # Run py.test against the unit tests.
+    # Print out package versions of dependencies
     session.run(
-        "py.test",
-        "--quiet",
-        f"--junitxml=unit_{session.python}_sponge_log.xml",
-        "--cov=google",
-        "--cov=tests/unit",
-        "--cov-append",
-        "--cov-config=.coveragerc",
-        "--cov-report=",
-        "--cov-fail-under=0",
-        os.path.join("tests", "unit"),
-        *session.posargs,
+        "python", "-c", "import google.protobuf; print(google.protobuf.__version__)"
     )
+    session.run("python", "-c", "import grpc; print(grpc.__version__)")
+    session.run("python", "-c", "import google.auth; print(google.auth.__version__)")
 
+    session.run(
+        "python", "-c", "import google.api_core; print(google.api_core.__version__)"
+    )
 
-def install_systemtest_dependencies(session, *constraints):
-
-    # Use pre-release gRPC for system tests.
-    session.install("--pre", "grpcio")
-
-    session.install(*SYSTEM_TEST_STANDARD_DEPENDENCIES, *constraints)
-
-    if SYSTEM_TEST_EXTERNAL_DEPENDENCIES:
-        session.install(*SYSTEM_TEST_EXTERNAL_DEPENDENCIES, *constraints)
-
-    if SYSTEM_TEST_LOCAL_DEPENDENCIES:
-        session.install("-e", *SYSTEM_TEST_LOCAL_DEPENDENCIES, *constraints)
-
-    if SYSTEM_TEST_DEPENDENCIES:
-        session.install("-e", *SYSTEM_TEST_DEPENDENCIES, *constraints)
+    # Run py.test against the unit tests in the downstream repository
+    with session.chdir(downstream_dir):
+        # Run py.test against the unit tests.
+        session.run(
+            "py.test",
+            "--quiet",
+            "--cov=google/cloud",
+            "--cov=tests/unit",
+            "--cov-append",
+            "--cov-config=.coveragerc",
+            "--cov-report=",
+            "--cov-fail-under=0",
+            os.path.join("tests", "unit"),
+            *session.posargs,
+            env={
+                "PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION": protobuf_implementation,
+            },
+        )
 
-    if SYSTEM_TEST_EXTRAS_BY_PYTHON:
-        extras = SYSTEM_TEST_EXTRAS_BY_PYTHON.get(session.python, [])
-    elif SYSTEM_TEST_EXTRAS:
-        extras = SYSTEM_TEST_EXTRAS
-    else:
-        extras = []
 
-    if extras:
-        session.install("-e", f".[{','.join(extras)}]", *constraints)
-    else:
-        session.install("-e", ".", *constraints)
+def install_prerelease_dependencies(session, constraints_path):
+    with open(constraints_path, encoding="utf-8") as constraints_file:
+        constraints_text = constraints_file.read()
+        # Ignore leading whitespace and comment lines.
+        constraints_deps = [
+            match.group(1)
+            for match in re.finditer(
+                r"^\s*(\S+)(?===\S+)", constraints_text, flags=re.MULTILINE
+            )
+        ]
+        session.install(*constraints_deps)
+        prerel_deps = [
+            "googleapis-common-protos",
+            "protobuf",
+            "six",
+            "grpcio",
+            "grpcio-status",
+            "google-api-core",
+            "google-auth",
+            "proto-plus",
+            "google-cloud-testutils",
+            # dependencies of google-cloud-testutils"
+            "click",
+        ]
+
+        for dep in prerel_deps:
+            session.install("--pre", "--no-deps", "--upgrade", dep)
+
+        # Remaining dependencies
+        other_deps = [
+            "requests",
+        ]
+        session.install(*other_deps)
 
 
 @nox.session(python=UNIT_TEST_PYTHON_VERSIONS)
 @nox.parametrize(
-    "library",
+    "library,prerelease,protobuf_implementation",
     [
-        ("google-cloud-python", "google-cloud-asset"),
+        (("google-cloud-python", "google-cloud-asset"), False, "python"),
+        (("google-cloud-python", "google-cloud-asset"), False, "upb"),
+        (("google-cloud-python", "google-cloud-asset"), False, "cpp"),
+        (("google-cloud-python", "google-cloud-asset"), True, "python"),
+        (("google-cloud-python", "google-cloud-asset"), True, "upb"),
+        (("google-cloud-python", "google-cloud-asset"), True, "cpp"),
     ],
-    ids=["asset"],
 )
-def integration_test(session, library):
+def unit_remote(session, library, prerelease, protobuf_implementation):
     """Run tests from a downstream libraries.
+
     To verify that any changes we make here will not break downstream libraries, clone
     a few and run their unit and system tests.
+
     NOTE: The unit and system test functions above are copied from the templates.
     They will need to be updated when the templates change.
+
+    * Pub/Sub: GAPIC with handwritten layer.
+    * Speech: Full GAPIC, has long running operations.
     """
-    package = ""
-    if type(library) == tuple:
-        library, package = library
+
+    if protobuf_implementation == "cpp" and session.python in ("3.11", "3.12"):
+        session.skip("cpp implementation is not supported in python 3.11+")
+
+    repository, package = library
     try:
-        session.run("git", "-C", library, "pull", external=True)
+        session.run("git", "-C", repository, "pull", external=True)
     except nox.command.CommandFailed:
         session.run(
             "git",
             "clone",
             "--single-branch",
-            f"https://github.com/googleapis/{library}",
+            f"https://github.com/googleapis/{repository}",
             external=True,
         )
 
-    session.cd(library)
-    if package:
-        session.cd(f"packages/{package}")
-    default(session)
-    # system tests are run 3.7 only
-    if session.python == "3.7":
-        system(session)
+    unit(
+        session=session,
+        repository=repository,
+        package=package,
+        prerelease=prerelease,
+        protobuf_implementation=protobuf_implementation,
+    )
+
+
+@nox.session(python=UNIT_TEST_PYTHON_VERSIONS)
+@nox.parametrize("protobuf_implementation", ["python", "upb", "cpp"])
+def unit_local(session, protobuf_implementation):
+    """Run tests in this local repo."""
+    # Install all test dependencies, then install this package in-place.
+
+    # TODO(https://github.com/googleapis/proto-plus-python/issues/389):
+    # Remove the 'cpp' implementation once support for Protobuf 3.x is dropped.
+    # The 'cpp' implementation requires Protobuf == 3.x however version 3.x
+    # does not support Python 3.11 and newer. The 'cpp' implementation
+    # must be excluded from the test matrix for these runtimes.
+    if protobuf_implementation == "cpp" and session.python in ("3.11", "3.12"):
+        session.skip("cpp implementation is not supported in python 3.11+")
+
+    constraints_path = str(
+        CURRENT_DIRECTORY / "testing" / f"constraints-{session.python}.txt"
+    )
+    session.install(
+        "mock",
+        "asyncmock",
+        "pytest",
+        "pytest-cov",
+        "pytest-asyncio",
+        "-c",
+        constraints_path,
+    )
+
+    session.install("-e", ".", "-c", constraints_path)
+
+    # Remove the 'cpp' implementation once support for Protobuf 3.x is dropped.
+    # The 'cpp' implementation requires Protobuf<4.
+    if protobuf_implementation == "cpp":
+        session.install("protobuf<4")
+
+    # Run py.test against the unit tests.
+    session.run(
+        "py.test",
+        "--quiet",
+        f"--junitxml=unit_{session.python}_sponge_log.xml",
+        "--cov=google",
+        "--cov=tests/unit",
+        "--cov-append",
+        "--cov-config=.coveragerc",
+        "--cov-report=",
+        "--cov-fail-under=0",
+        os.path.join("tests", "unit"),
+        *session.posargs,
+        env={
+            "PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION": protobuf_implementation,
+        },
+    )
 
 
 @nox.session(python="3.8")
 def generate_protos(session):
     """Generates the protos using protoc.
+
+    This session but be last to avoid overwriting the protos used in CI runs.
+
     Some notes on the `google` directory:
     1. The `_pb2.py` files are produced by protoc.
     2. The .proto files are non-functional but are left in the repository
@@ -262,60 +276,12 @@ def generate_protos(session):
        If a new subdirectory is added, you will need to create more `__init__.py`
        files.
     """
-    session.install(GRPCIO_TOOLS_VERSION)
-    protos = [str(p) for p in (pathlib.Path(".").glob("google/**/*.proto"))]
 
+    session.install(GRPCIO_TOOLS_VERSION)
+    protos = [str(p) for p in (Path(".").glob("google/**/*.proto"))]
     session.run(
-        "python",
-        "-m",
-        "grpc_tools.protoc",
-        "--proto_path=.",
-        "--python_out=.",
-        *protos,
-    )
-
-
-@nox.session(python=SYSTEM_TEST_PYTHON_VERSIONS)
-def system(session):
-    """Run the system test suite."""
-    constraints_path = str(
-        CURRENT_DIRECTORY / "testing" / f"constraints-{session.python}.txt"
+        "python", "-m", "grpc_tools.protoc", "--proto_path=.", "--python_out=.", *protos
     )
-    system_test_path = os.path.join("tests", "system.py")
-    system_test_folder_path = os.path.join("tests", "system")
-
-    # Check the value of `RUN_SYSTEM_TESTS` env var. It defaults to true.
-    if os.environ.get("RUN_SYSTEM_TESTS", "true") == "false":
-        session.skip("RUN_SYSTEM_TESTS is set to false, skipping")
-    # Install pyopenssl for mTLS testing.
-    if os.environ.get("GOOGLE_API_USE_CLIENT_CERTIFICATE", "false") == "true":
-        session.install("pyopenssl")
-
-    system_test_exists = os.path.exists(system_test_path)
-    system_test_folder_exists = os.path.exists(system_test_folder_path)
-    # Sanity check: only run tests if found.
-    if not system_test_exists and not system_test_folder_exists:
-        session.skip("System tests were not found")
-
-    install_systemtest_dependencies(session, "-c", constraints_path)
-
-    # Run py.test against the system tests.
-    if system_test_exists:
-        session.run(
-            "py.test",
-            "--quiet",
-            f"--junitxml=system_{session.python}_sponge_log.xml",
-            system_test_path,
-            *session.posargs,
-        )
-    if system_test_folder_exists:
-        session.run(
-            "py.test",
-            "--quiet",
-            f"--junitxml=system_{session.python}_sponge_log.xml",
-            system_test_folder_path,
-            *session.posargs,
-        )
 
 
 @nox.session(python="3.9")
@@ -399,7 +365,16 @@ def docfx(session):
     )
 
 
-@nox.session(python=UNIT_TEST_PYTHON_VERSIONS)
-def unit(session):
-    """Run the unit test suite."""
-    default(session)
+@nox.session(python="3.8")
+def lint(session):
+    """Run linters.
+
+    Returns a failure if the linters find linting errors or sufficiently
+    serious code quality issues.
+    """
+    session.install("flake8", BLACK_VERSION)
+    session.run(
+        "black",
+        "--check",
+        *LINT_PATHS,
+    )
diff --git a/setup.py b/setup.py
index 9fefb46..72d57d3 100644
--- a/setup.py
+++ b/setup.py
@@ -30,7 +30,7 @@
 release_status = "Development Status :: 4 - Beta"
 dependencies = [
     "google-api-core[grpc] >= 1.34.1, <3.0.0dev,!=2.0.*,!=2.1.*,!=2.2.*,!=2.3.*,!=2.4.*,!=2.5.*,!=2.6.*,!=2.7.*,!=2.8.*,!=2.9.*,!=2.10.*",
-    "protobuf>=3.19.5,<5.0.0dev,!=3.20.0,!=3.20.1,!=4.21.1,!=4.21.2,!=4.21.3,!=4.21.4,!=4.21.5",
+    "protobuf>=3.20.2,<6.0.0dev,!=4.21.1,!=4.21.2,!=4.21.3,!=4.21.4,!=4.21.5",
 ]
 
 # Setup boilerplate below this line.
diff --git a/testing/constraints-3.7.txt b/testing/constraints-3.7.txt
index 7a1c665..5dccad8 100644
--- a/testing/constraints-3.7.txt
+++ b/testing/constraints-3.7.txt
@@ -5,5 +5,5 @@
 #
 # e.g., if setup.py has "foo >= 1.14.0, < 2.0.0dev",
 # Then this file should have foo==1.14.0
-protobuf==3.19.5
+protobuf==3.20.2
 google-api-core==1.34.1