diff --git a/docs/reference/google.auth.crypt.base.rst b/docs/reference/google.auth.crypt.base.rst
deleted file mode 100644
index a899650..0000000
--- a/docs/reference/google.auth.crypt.base.rst
+++ /dev/null
@@ -1,7 +0,0 @@
-google.auth.crypt.base module
-=============================
-
-.. automodule:: google.auth.crypt.base
-   :members:
-   :inherited-members:
-   :show-inheritance:
diff --git a/docs/reference/google.auth.crypt.rsa.rst b/docs/reference/google.auth.crypt.rsa.rst
deleted file mode 100644
index 7060b03..0000000
--- a/docs/reference/google.auth.crypt.rsa.rst
+++ /dev/null
@@ -1,7 +0,0 @@
-google.auth.crypt.rsa module
-============================
-
-.. automodule:: google.auth.crypt.rsa
-   :members:
-   :inherited-members:
-   :show-inheritance:
diff --git a/google/oauth2/credentials.py b/google/oauth2/credentials.py
index 422c8ab..3a32c06 100644
--- a/google/oauth2/credentials.py
+++ b/google/oauth2/credentials.py
@@ -58,7 +58,6 @@
         client_id=None,
         client_secret=None,
         scopes=None,
-        quota_project_id=None,
     ):
         """
         Args:
@@ -82,9 +81,6 @@
                 token if refresh information is provided (e.g. The refresh
                 token scopes are a superset of this or contain a wild card
                 scope like 'https://www.googleapis.com/auth/any-api').
-            quota_project_id (Optional[str]): The project ID used for quota and billing.
-                This project may be different from the project used to
-                create the credentials.
         """
         super(Credentials, self).__init__()
         self.token = token
@@ -94,7 +90,6 @@
         self._token_uri = token_uri
         self._client_id = client_id
         self._client_secret = client_secret
-        self._quota_project_id = quota_project_id
 
     @property
     def refresh_token(self):
@@ -129,11 +124,6 @@
         return self._client_secret
 
     @property
-    def quota_project_id(self):
-        """Optional[str]: The project to use for quota and billing purposes."""
-        return self._quota_project_id
-
-    @property
     def requires_scopes(self):
         """False: OAuth 2.0 credentials have their scopes set when
         the initial token is requested and can not be changed."""
@@ -179,12 +169,6 @@
                     )
                 )
 
-    @_helpers.copy_docstring(credentials.Credentials)
-    def apply(self, headers, token=None):
-        super(Credentials, self).apply(headers, token=token)
-        if self.quota_project_id is not None:
-            headers["x-goog-user-project"] = self.quota_project_id
-
     @classmethod
     def from_authorized_user_info(cls, info, scopes=None):
         """Creates a Credentials instance from parsed authorized user info.
@@ -218,9 +202,6 @@
             scopes=scopes,
             client_id=info["client_id"],
             client_secret=info["client_secret"],
-            quota_project_id=info.get(
-                "quota_project_id"
-            ),  # quota project may not exist
         )
 
     @classmethod
diff --git a/tests/oauth2/test_credentials.py b/tests/oauth2/test_credentials.py
index 59031d7..bb70f15 100644
--- a/tests/oauth2/test_credentials.py
+++ b/tests/oauth2/test_credentials.py
@@ -294,33 +294,6 @@
         # expired.)
         assert creds.valid
 
-    def test_apply_with_quota_project_id(self):
-        creds = credentials.Credentials(
-            token="token",
-            refresh_token=self.REFRESH_TOKEN,
-            token_uri=self.TOKEN_URI,
-            client_id=self.CLIENT_ID,
-            client_secret=self.CLIENT_SECRET,
-            quota_project_id="quota-project-123",
-        )
-
-        headers = {}
-        creds.apply(headers)
-        assert headers["x-goog-user-project"] == "quota-project-123"
-
-    def test_apply_with_no_quota_project_id(self):
-        creds = credentials.Credentials(
-            token="token",
-            refresh_token=self.REFRESH_TOKEN,
-            token_uri=self.TOKEN_URI,
-            client_id=self.CLIENT_ID,
-            client_secret=self.CLIENT_SECRET,
-        )
-
-        headers = {}
-        creds.apply(headers)
-        assert "x-goog-user-project" not in headers
-
     def test_from_authorized_user_info(self):
         info = AUTH_USER_INFO.copy()