diff --git a/.github/.OwlBot.lock.yaml b/.github/.OwlBot.lock.yaml
index 2aefd0e9..5d9542b1 100644
--- a/.github/.OwlBot.lock.yaml
+++ b/.github/.OwlBot.lock.yaml
@@ -13,5 +13,5 @@
 # limitations under the License.
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-python:latest
-  digest: sha256:97b671488ad548ef783a452a9e1276ac10f144d5ae56d98cc4bf77ba504082b4
-# created: 2024-02-06T03:20:16.660474034Z
+  digest: sha256:3741fd1f5f5150378563c76afa06bcc12777b5fe54c5ee01115218f83872134f
+# created: 2024-03-15T16:26:15.743347415Z
diff --git a/.kokoro/build.sh b/.kokoro/build.sh
index 05618cbc..b3c0ce5e 100755
--- a/.kokoro/build.sh
+++ b/.kokoro/build.sh
@@ -33,13 +33,6 @@ export GOOGLE_APPLICATION_CREDENTIALS=${KOKORO_GFILE_DIR}/service-account.json
 # Setup project id.
 export PROJECT_ID=$(cat "${KOKORO_GFILE_DIR}/project-id.json")
 
-# Remove old nox
-python3 -m pip uninstall --yes --quiet nox-automation
-
-# Install nox
-python3 -m pip install --upgrade --quiet nox
-python3 -m nox --version
-
 # If this is a continuous build, send the test log to the FlakyBot.
 # See https://github.com/googleapis/repo-automation-bots/tree/main/packages/flakybot.
 if [[ $KOKORO_BUILD_ARTIFACTS_SUBDIR = *"continuous"* ]]; then
diff --git a/.kokoro/docker/docs/Dockerfile b/.kokoro/docker/docs/Dockerfile
index 8e39a2cc..bdaf39fe 100644
--- a/.kokoro/docker/docs/Dockerfile
+++ b/.kokoro/docker/docs/Dockerfile
@@ -80,4 +80,8 @@ RUN wget -O /tmp/get-pip.py 'https://bootstrap.pypa.io/get-pip.py' \
 # Test pip
 RUN python3 -m pip
 
+# Install build requirements
+COPY requirements.txt /requirements.txt
+RUN python3 -m pip install --require-hashes -r requirements.txt
+
 CMD ["python3.8"]
diff --git a/.kokoro/docker/docs/requirements.in b/.kokoro/docker/docs/requirements.in
new file mode 100644
index 00000000..816817c6
--- /dev/null
+++ b/.kokoro/docker/docs/requirements.in
@@ -0,0 +1 @@
+nox
diff --git a/.kokoro/docker/docs/requirements.txt b/.kokoro/docker/docs/requirements.txt
new file mode 100644
index 00000000..0e5d70f2
--- /dev/null
+++ b/.kokoro/docker/docs/requirements.txt
@@ -0,0 +1,38 @@
+#
+# This file is autogenerated by pip-compile with Python 3.9
+# by the following command:
+#
+#    pip-compile --allow-unsafe --generate-hashes requirements.in
+#
+argcomplete==3.2.3 \
+    --hash=sha256:bf7900329262e481be5a15f56f19736b376df6f82ed27576fa893652c5de6c23 \
+    --hash=sha256:c12355e0494c76a2a7b73e3a59b09024ca0ba1e279fb9ed6c1b82d5b74b6a70c
+    # via nox
+colorlog==6.8.2 \
+    --hash=sha256:3e3e079a41feb5a1b64f978b5ea4f46040a94f11f0e8bbb8261e3dbbeca64d44 \
+    --hash=sha256:4dcbb62368e2800cb3c5abd348da7e53f6c362dda502ec27c560b2e58a66bd33
+    # via nox
+distlib==0.3.8 \
+    --hash=sha256:034db59a0b96f8ca18035f36290806a9a6e6bd9d1ff91e45a7f172eb17e51784 \
+    --hash=sha256:1530ea13e350031b6312d8580ddb6b27a104275a31106523b8f123787f494f64
+    # via virtualenv
+filelock==3.13.1 \
+    --hash=sha256:521f5f56c50f8426f5e03ad3b281b490a87ef15bc6c526f168290f0c7148d44e \
+    --hash=sha256:57dbda9b35157b05fb3e58ee91448612eb674172fab98ee235ccb0b5bee19a1c
+    # via virtualenv
+nox==2024.3.2 \
+    --hash=sha256:e53514173ac0b98dd47585096a55572fe504fecede58ced708979184d05440be \
+    --hash=sha256:f521ae08a15adbf5e11f16cb34e8d0e6ea521e0b92868f684e91677deb974553
+    # via -r requirements.in
+packaging==24.0 \
+    --hash=sha256:2ddfb553fdf02fb784c234c7ba6ccc288296ceabec964ad2eae3777778130bc5 \
+    --hash=sha256:eb82c5e3e56209074766e6885bb04b8c38a0c015d0a30036ebe7ece34c9989e9
+    # via nox
+platformdirs==4.2.0 \
+    --hash=sha256:0614df2a2f37e1a662acbd8e2b25b92ccf8632929bc6d43467e17fe89c75e068 \
+    --hash=sha256:ef0cc731df711022c174543cb70a9b5bd22e5a9337c8624ef2c2ceb8ddad8768
+    # via virtualenv
+virtualenv==20.25.1 \
+    --hash=sha256:961c026ac520bac5f69acb8ea063e8a4f071bcc9457b9c1f28f6b085c511583a \
+    --hash=sha256:e08e13ecdca7a0bd53798f356d5831434afa5b07b93f0abdf0797b7a06ffe197
+    # via nox
diff --git a/.kokoro/requirements.txt b/.kokoro/requirements.txt
index 8c11c9f3..bda8e38c 100644
--- a/.kokoro/requirements.txt
+++ b/.kokoro/requirements.txt
@@ -93,39 +93,39 @@ colorlog==6.7.0 \
     # via
     #   gcp-docuploader
     #   nox
-cryptography==42.0.0 \
-    --hash=sha256:0a68bfcf57a6887818307600c3c0ebc3f62fbb6ccad2240aa21887cda1f8df1b \
-    --hash=sha256:146e971e92a6dd042214b537a726c9750496128453146ab0ee8971a0299dc9bd \
-    --hash=sha256:14e4b909373bc5bf1095311fa0f7fcabf2d1a160ca13f1e9e467be1ac4cbdf94 \
-    --hash=sha256:206aaf42e031b93f86ad60f9f5d9da1b09164f25488238ac1dc488334eb5e221 \
-    --hash=sha256:3005166a39b70c8b94455fdbe78d87a444da31ff70de3331cdec2c568cf25b7e \
-    --hash=sha256:324721d93b998cb7367f1e6897370644751e5580ff9b370c0a50dc60a2003513 \
-    --hash=sha256:33588310b5c886dfb87dba5f013b8d27df7ffd31dc753775342a1e5ab139e59d \
-    --hash=sha256:35cf6ed4c38f054478a9df14f03c1169bb14bd98f0b1705751079b25e1cb58bc \
-    --hash=sha256:3ca482ea80626048975360c8e62be3ceb0f11803180b73163acd24bf014133a0 \
-    --hash=sha256:56ce0c106d5c3fec1038c3cca3d55ac320a5be1b44bf15116732d0bc716979a2 \
-    --hash=sha256:5a217bca51f3b91971400890905a9323ad805838ca3fa1e202a01844f485ee87 \
-    --hash=sha256:678cfa0d1e72ef41d48993a7be75a76b0725d29b820ff3cfd606a5b2b33fda01 \
-    --hash=sha256:69fd009a325cad6fbfd5b04c711a4da563c6c4854fc4c9544bff3088387c77c0 \
-    --hash=sha256:6cf9b76d6e93c62114bd19485e5cb003115c134cf9ce91f8ac924c44f8c8c3f4 \
-    --hash=sha256:74f18a4c8ca04134d2052a140322002fef535c99cdbc2a6afc18a8024d5c9d5b \
-    --hash=sha256:85f759ed59ffd1d0baad296e72780aa62ff8a71f94dc1ab340386a1207d0ea81 \
-    --hash=sha256:87086eae86a700307b544625e3ba11cc600c3c0ef8ab97b0fda0705d6db3d4e3 \
-    --hash=sha256:8814722cffcfd1fbd91edd9f3451b88a8f26a5fd41b28c1c9193949d1c689dc4 \
-    --hash=sha256:8fedec73d590fd30c4e3f0d0f4bc961aeca8390c72f3eaa1a0874d180e868ddf \
-    --hash=sha256:9515ea7f596c8092fdc9902627e51b23a75daa2c7815ed5aa8cf4f07469212ec \
-    --hash=sha256:988b738f56c665366b1e4bfd9045c3efae89ee366ca3839cd5af53eaa1401bce \
-    --hash=sha256:a2a8d873667e4fd2f34aedab02ba500b824692c6542e017075a2efc38f60a4c0 \
-    --hash=sha256:bd7cf7a8d9f34cc67220f1195884151426ce616fdc8285df9054bfa10135925f \
-    --hash=sha256:bdce70e562c69bb089523e75ef1d9625b7417c6297a76ac27b1b8b1eb51b7d0f \
-    --hash=sha256:be14b31eb3a293fc6e6aa2807c8a3224c71426f7c4e3639ccf1a2f3ffd6df8c3 \
-    --hash=sha256:be41b0c7366e5549265adf2145135dca107718fa44b6e418dc7499cfff6b4689 \
-    --hash=sha256:c310767268d88803b653fffe6d6f2f17bb9d49ffceb8d70aed50ad45ea49ab08 \
-    --hash=sha256:c58115384bdcfe9c7f644c72f10f6f42bed7cf59f7b52fe1bf7ae0a622b3a139 \
-    --hash=sha256:c640b0ef54138fde761ec99a6c7dc4ce05e80420262c20fa239e694ca371d434 \
-    --hash=sha256:ca20550bb590db16223eb9ccc5852335b48b8f597e2f6f0878bbfd9e7314eb17 \
-    --hash=sha256:d97aae66b7de41cdf5b12087b5509e4e9805ed6f562406dfcf60e8481a9a28f8 \
-    --hash=sha256:e9326ca78111e4c645f7e49cbce4ed2f3f85e17b61a563328c85a5208cf34440
+cryptography==42.0.4 \
+    --hash=sha256:01911714117642a3f1792c7f376db572aadadbafcd8d75bb527166009c9f1d1b \
+    --hash=sha256:0e89f7b84f421c56e7ff69f11c441ebda73b8a8e6488d322ef71746224c20fce \
+    --hash=sha256:12d341bd42cdb7d4937b0cabbdf2a94f949413ac4504904d0cdbdce4a22cbf88 \
+    --hash=sha256:15a1fb843c48b4a604663fa30af60818cd28f895572386e5f9b8a665874c26e7 \
+    --hash=sha256:1cdcdbd117681c88d717437ada72bdd5be9de117f96e3f4d50dab3f59fd9ab20 \
+    --hash=sha256:1df6fcbf60560d2113b5ed90f072dc0b108d64750d4cbd46a21ec882c7aefce9 \
+    --hash=sha256:3c6048f217533d89f2f8f4f0fe3044bf0b2090453b7b73d0b77db47b80af8dff \
+    --hash=sha256:3e970a2119507d0b104f0a8e281521ad28fc26f2820687b3436b8c9a5fcf20d1 \
+    --hash=sha256:44a64043f743485925d3bcac548d05df0f9bb445c5fcca6681889c7c3ab12764 \
+    --hash=sha256:4e36685cb634af55e0677d435d425043967ac2f3790ec652b2b88ad03b85c27b \
+    --hash=sha256:5f8907fcf57392cd917892ae83708761c6ff3c37a8e835d7246ff0ad251d9298 \
+    --hash=sha256:69b22ab6506a3fe483d67d1ed878e1602bdd5912a134e6202c1ec672233241c1 \
+    --hash=sha256:6bfadd884e7280df24d26f2186e4e07556a05d37393b0f220a840b083dc6a824 \
+    --hash=sha256:6d0fbe73728c44ca3a241eff9aefe6496ab2656d6e7a4ea2459865f2e8613257 \
+    --hash=sha256:6ffb03d419edcab93b4b19c22ee80c007fb2d708429cecebf1dd3258956a563a \
+    --hash=sha256:810bcf151caefc03e51a3d61e53335cd5c7316c0a105cc695f0959f2c638b129 \
+    --hash=sha256:831a4b37accef30cccd34fcb916a5d7b5be3cbbe27268a02832c3e450aea39cb \
+    --hash=sha256:887623fe0d70f48ab3f5e4dbf234986b1329a64c066d719432d0698522749929 \
+    --hash=sha256:a0298bdc6e98ca21382afe914c642620370ce0470a01e1bef6dd9b5354c36854 \
+    --hash=sha256:a1327f280c824ff7885bdeef8578f74690e9079267c1c8bd7dc5cc5aa065ae52 \
+    --hash=sha256:c1f25b252d2c87088abc8bbc4f1ecbf7c919e05508a7e8628e6875c40bc70923 \
+    --hash=sha256:c3a5cbc620e1e17009f30dd34cb0d85c987afd21c41a74352d1719be33380885 \
+    --hash=sha256:ce8613beaffc7c14f091497346ef117c1798c202b01153a8cc7b8e2ebaaf41c0 \
+    --hash=sha256:d2a27aca5597c8a71abbe10209184e1a8e91c1fd470b5070a2ea60cafec35bcd \
+    --hash=sha256:dad9c385ba8ee025bb0d856714f71d7840020fe176ae0229de618f14dae7a6e2 \
+    --hash=sha256:db4b65b02f59035037fde0998974d84244a64c3265bdef32a827ab9b63d61b18 \
+    --hash=sha256:e09469a2cec88fb7b078e16d4adec594414397e8879a4341c6ace96013463d5b \
+    --hash=sha256:e53dc41cda40b248ebc40b83b31516487f7db95ab8ceac1f042626bc43a2f992 \
+    --hash=sha256:f1e85a178384bf19e36779d91ff35c7617c885da487d689b05c1366f9933ad74 \
+    --hash=sha256:f47be41843200f7faec0683ad751e5ef11b9a56a220d57f300376cd8aba81660 \
+    --hash=sha256:fb0cef872d8193e487fc6bdb08559c3aa41b659a7d9be48b2e10747f47863925 \
+    --hash=sha256:ffc73996c4fca3d2b6c1c8c12bfd3ad00def8621da24f547626bf06441400449
     # via
     #   gcp-releasetool
     #   secretstorage
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 28cc08a0..b1643772 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,19 @@
 
 [1]: https://pypi.org/project/google-api-core/#history
 
+## [2.18.0](https://github.com/googleapis/python-api-core/compare/v2.17.1...v2.18.0) (2024-03-20)
+
+
+### Features
+
+* Add common logic for supporting universe domain ([#621](https://github.com/googleapis/python-api-core/issues/621)) ([94f2ca3](https://github.com/googleapis/python-api-core/commit/94f2ca3b4d094e6e10154634d3463d07ebea2035))
+
+
+### Bug Fixes
+
+* Add _registered_method to grpc ChannelStub ([#614](https://github.com/googleapis/python-api-core/issues/614)) ([5eaaea8](https://github.com/googleapis/python-api-core/commit/5eaaea8a989f8bdbdb5fbc95a155a20837c87f42))
+* **deps:** Require proto-plus >= 1.22.3 ([#626](https://github.com/googleapis/python-api-core/issues/626)) ([4fed37c](https://github.com/googleapis/python-api-core/commit/4fed37cbc32122f156e38250b5fa8b2b08a787a1))
+
 ## [2.17.1](https://github.com/googleapis/python-api-core/compare/v2.17.0...v2.17.1) (2024-02-13)
 
 
diff --git a/google/api_core/grpc_helpers.py b/google/api_core/grpc_helpers.py
index 21c7315f..1dcbb8b9 100644
--- a/google/api_core/grpc_helpers.py
+++ b/google/api_core/grpc_helpers.py
@@ -569,20 +569,42 @@ def __getattr__(self, key):
         except KeyError:
             raise AttributeError
 
-    def unary_unary(self, method, request_serializer=None, response_deserializer=None):
+    def unary_unary(
+        self,
+        method,
+        request_serializer=None,
+        response_deserializer=None,
+        _registered_method=False,
+    ):
         """grpc.Channel.unary_unary implementation."""
         return self._stub_for_method(method)
 
-    def unary_stream(self, method, request_serializer=None, response_deserializer=None):
+    def unary_stream(
+        self,
+        method,
+        request_serializer=None,
+        response_deserializer=None,
+        _registered_method=False,
+    ):
         """grpc.Channel.unary_stream implementation."""
         return self._stub_for_method(method)
 
-    def stream_unary(self, method, request_serializer=None, response_deserializer=None):
+    def stream_unary(
+        self,
+        method,
+        request_serializer=None,
+        response_deserializer=None,
+        _registered_method=False,
+    ):
         """grpc.Channel.stream_unary implementation."""
         return self._stub_for_method(method)
 
     def stream_stream(
-        self, method, request_serializer=None, response_deserializer=None
+        self,
+        method,
+        request_serializer=None,
+        response_deserializer=None,
+        _registered_method=False,
     ):
         """grpc.Channel.stream_stream implementation."""
         return self._stub_for_method(method)
diff --git a/google/api_core/universe.py b/google/api_core/universe.py
new file mode 100644
index 00000000..35669642
--- /dev/null
+++ b/google/api_core/universe.py
@@ -0,0 +1,82 @@
+# Copyright 2024 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Helpers for universe domain."""
+
+from typing import Any, Optional
+
+DEFAULT_UNIVERSE = "googleapis.com"
+
+
+class EmptyUniverseError(ValueError):
+    def __init__(self):
+        message = "Universe Domain cannot be an empty string."
+        super().__init__(message)
+
+
+class UniverseMismatchError(ValueError):
+    def __init__(self, client_universe, credentials_universe):
+        message = (
+            f"The configured universe domain ({client_universe}) does not match the universe domain "
+            f"found in the credentials ({credentials_universe}). "
+            "If you haven't configured the universe domain explicitly, "
+            f"`{DEFAULT_UNIVERSE}` is the default."
+        )
+        super().__init__(message)
+
+
+def determine_domain(
+    client_universe_domain: Optional[str], universe_domain_env: Optional[str]
+) -> str:
+    """Return the universe domain used by the client.
+
+    Args:
+        client_universe_domain (Optional[str]): The universe domain configured via the client options.
+        universe_domain_env (Optional[str]): The universe domain configured via the
+        "GOOGLE_CLOUD_UNIVERSE_DOMAIN" environment variable.
+
+    Returns:
+        str: The universe domain to be used by the client.
+
+    Raises:
+        ValueError: If the universe domain is an empty string.
+    """
+    universe_domain = DEFAULT_UNIVERSE
+    if client_universe_domain is not None:
+        universe_domain = client_universe_domain
+    elif universe_domain_env is not None:
+        universe_domain = universe_domain_env
+    if len(universe_domain.strip()) == 0:
+        raise EmptyUniverseError
+    return universe_domain
+
+
+def compare_domains(client_universe: str, credentials: Any) -> bool:
+    """Returns True iff the universe domains used by the client and credentials match.
+
+    Args:
+        client_universe (str): The universe domain configured via the client options.
+        credentials Any: The credentials being used in the client.
+
+    Returns:
+        bool: True iff client_universe matches the universe in credentials.
+
+    Raises:
+        ValueError: when client_universe does not match the universe in credentials.
+    """
+    credentials_universe = getattr(credentials, "universe_domain", DEFAULT_UNIVERSE)
+
+    if client_universe != credentials_universe:
+        raise UniverseMismatchError(client_universe, credentials_universe)
+    return True
diff --git a/google/api_core/version.py b/google/api_core/version.py
index 3fd9f000..a613e5ea 100644
--- a/google/api_core/version.py
+++ b/google/api_core/version.py
@@ -12,4 +12,4 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-__version__ = "2.17.1"
+__version__ = "2.18.0"
diff --git a/noxfile.py b/noxfile.py
index a6fdecde..2c7ec6c7 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -44,16 +44,6 @@
 ]
 
 
-def _greater_or_equal_than_37(version_string):
-    tokens = version_string.split(".")
-    for i, token in enumerate(tokens):
-        try:
-            tokens[i] = int(token)
-        except ValueError:
-            pass
-    return tokens >= [3, 7]
-
-
 @nox.session(python=DEFAULT_PYTHON_VERSION)
 def lint(session):
     """Run linters.
@@ -129,16 +119,13 @@ def default(session, install_grpc=True):
         ),
     ]
 
-    # Inject AsyncIO content and proto-plus, if version >= 3.7.
-    # proto-plus is needed for a field mask test in test_protobuf_helpers.py
-    if _greater_or_equal_than_37(session.python):
-        session.install("asyncmock", "pytest-asyncio", "proto-plus")
+    session.install("asyncmock", "pytest-asyncio")
 
-        # Having positional arguments means the user wants to run specific tests.
-        # Best not to add additional tests to that list.
-        if not session.posargs:
-            pytest_args.append("--cov=tests.asyncio")
-            pytest_args.append(os.path.join("tests", "asyncio"))
+    # Having positional arguments means the user wants to run specific tests.
+    # Best not to add additional tests to that list.
+    if not session.posargs:
+        pytest_args.append("--cov=tests.asyncio")
+        pytest_args.append(os.path.join("tests", "asyncio"))
 
     session.run(*pytest_args)
 
diff --git a/setup.py b/setup.py
index 47a3c203..47e0b454 100644
--- a/setup.py
+++ b/setup.py
@@ -31,6 +31,7 @@
 dependencies = [
     "googleapis-common-protos >= 1.56.2, < 2.0.dev0",
     "protobuf>=3.19.5,<5.0.0.dev0,!=3.20.0,!=3.20.1,!=4.21.0,!=4.21.1,!=4.21.2,!=4.21.3,!=4.21.4,!=4.21.5",
+    "proto-plus >= 1.22.3, <2.0.0dev",
     "google-auth >= 2.14.1, < 3.0.dev0",
     "requests >= 2.18.0, < 3.0.0.dev0",
 ]
diff --git a/testing/constraints-3.7.txt b/testing/constraints-3.7.txt
index ec041297..fcc9831f 100644
--- a/testing/constraints-3.7.txt
+++ b/testing/constraints-3.7.txt
@@ -13,3 +13,4 @@ packaging==14.3
 grpcio==1.33.2
 grpcio-status==1.33.2
 grpcio-gcp==0.2.2
+proto-plus==1.22.3
diff --git a/tests/unit/test_protobuf_helpers.py b/tests/unit/test_protobuf_helpers.py
index 3df45df5..5b2c6dfd 100644
--- a/tests/unit/test_protobuf_helpers.py
+++ b/tests/unit/test_protobuf_helpers.py
@@ -12,8 +12,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import sys
-
 import pytest
 
 from google.api import http_pb2
@@ -476,11 +474,6 @@ def test_field_mask_different_level_diffs():
     ]
 
 
-@pytest.mark.skipif(
-    sys.version_info.major == 2,
-    reason="Field names with trailing underscores can only be created"
-    "through proto-plus, which is Python 3 only.",
-)
 def test_field_mask_ignore_trailing_underscore():
     import proto
 
@@ -496,11 +489,6 @@ class Foo(proto.Message):
     ]
 
 
-@pytest.mark.skipif(
-    sys.version_info.major == 2,
-    reason="Field names with trailing underscores can only be created"
-    "through proto-plus, which is Python 3 only.",
-)
 def test_field_mask_ignore_trailing_underscore_with_nesting():
     import proto
 
diff --git a/tests/unit/test_universe.py b/tests/unit/test_universe.py
new file mode 100644
index 00000000..214e00ac
--- /dev/null
+++ b/tests/unit/test_universe.py
@@ -0,0 +1,63 @@
+# Copyright 2024 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import pytest
+from google.api_core import universe
+
+
+class _Fake_Credentials:
+    def __init__(self, universe_domain=None):
+        if universe_domain:
+            self.universe_domain = universe_domain
+
+
+def test_determine_domain():
+    domain_client = "foo.com"
+    domain_env = "bar.com"
+
+    assert universe.determine_domain(domain_client, domain_env) == domain_client
+    assert universe.determine_domain(None, domain_env) == domain_env
+    assert universe.determine_domain(domain_client, None) == domain_client
+    assert universe.determine_domain(None, None) == universe.DEFAULT_UNIVERSE
+
+    with pytest.raises(universe.EmptyUniverseError):
+        universe.determine_domain("", None)
+
+    with pytest.raises(universe.EmptyUniverseError):
+        universe.determine_domain(None, "")
+
+
+def test_compare_domains():
+    fake_domain = "foo.com"
+    another_fake_domain = "bar.com"
+
+    assert universe.compare_domains(universe.DEFAULT_UNIVERSE, _Fake_Credentials())
+    assert universe.compare_domains(fake_domain, _Fake_Credentials(fake_domain))
+
+    with pytest.raises(universe.UniverseMismatchError) as excinfo:
+        universe.compare_domains(
+            universe.DEFAULT_UNIVERSE, _Fake_Credentials(fake_domain)
+        )
+    assert str(excinfo.value).find(universe.DEFAULT_UNIVERSE) >= 0
+    assert str(excinfo.value).find(fake_domain) >= 0
+
+    with pytest.raises(universe.UniverseMismatchError) as excinfo:
+        universe.compare_domains(fake_domain, _Fake_Credentials())
+    assert str(excinfo.value).find(fake_domain) >= 0
+    assert str(excinfo.value).find(universe.DEFAULT_UNIVERSE) >= 0
+
+    with pytest.raises(universe.UniverseMismatchError) as excinfo:
+        universe.compare_domains(fake_domain, _Fake_Credentials(another_fake_domain))
+    assert str(excinfo.value).find(fake_domain) >= 0
+    assert str(excinfo.value).find(another_fake_domain) >= 0