diff --git a/google/auth/jwt.py b/google/auth/jwt.py
index 412f122..b1eb5fb 100644
--- a/google/auth/jwt.py
+++ b/google/auth/jwt.py
@@ -46,13 +46,17 @@
 import datetime
 import json
 
+import cachetools
+from six.moves import urllib
+
 from google.auth import _helpers
 from google.auth import _service_account_info
 from google.auth import crypt
+from google.auth import exceptions
 import google.auth.credentials
 
-
 _DEFAULT_TOKEN_LIFETIME_SECS = 3600  # 1 hour in seconds
+_DEFAULT_MAX_CACHE_SIZE = 10
 
 
 def encode(signer, payload, header=None, key_id=None):
@@ -316,10 +320,10 @@
         self._audience = audience
         self._token_lifetime = token_lifetime
 
-        if additional_claims is not None:
-            self._additional_claims = additional_claims
-        else:
-            self._additional_claims = {}
+        if additional_claims is None:
+            additional_claims = {}
+
+        self._additional_claims = additional_claims
 
     @classmethod
     def _from_signer_and_info(cls, signer, info, **kwargs):
@@ -343,8 +347,7 @@
 
     @classmethod
     def from_service_account_info(cls, info, **kwargs):
-        """Creates a Credentials instance from a dictionary containing service
-        account info in Google format.
+        """Creates an Credentials instance from a dictionary.
 
         Args:
             info (Mapping[str, str]): The service account info in Google
@@ -487,3 +490,266 @@
     @_helpers.copy_docstring(google.auth.credentials.Signing)
     def signer(self):
         return self._signer
+
+
+class OnDemandCredentials(
+        google.auth.credentials.Signing,
+        google.auth.credentials.Credentials):
+    """On-demand JWT credentials.
+
+    Like :class:`Credentials`, this class uses a JWT as the bearer token for
+    authentication. However, this class does not require the audience at
+    construction time. Instead, it will generate a new token on-demand for
+    each request using the request URI as the audience. It caches tokens
+    so that multiple requests to the same URI do not incur the overhead
+    of generating a new token every time.
+
+    This behavior is especially useful for `gRPC`_ clients. A gRPC service may
+    have multiple audience and gRPC clients may not know all of the audiences
+    required for accessing a particular service. With these credentials,
+    no knowledge of the audiences is required ahead of time.
+
+    .. _grpc: http://www.grpc.io/
+    """
+
+    def __init__(self, signer, issuer, subject,
+                 additional_claims=None,
+                 token_lifetime=_DEFAULT_TOKEN_LIFETIME_SECS,
+                 max_cache_size=_DEFAULT_MAX_CACHE_SIZE):
+        """
+        Args:
+            signer (google.auth.crypt.Signer): The signer used to sign JWTs.
+            issuer (str): The `iss` claim.
+            subject (str): The `sub` claim.
+            additional_claims (Mapping[str, str]): Any additional claims for
+                the JWT payload.
+            token_lifetime (int): The amount of time in seconds for
+                which the token is valid. Defaults to 1 hour.
+            max_cache_size (int): The maximum number of JWT tokens to keep in
+                cache. Tokens are cached using :class:`cachetools.LRUCache`.
+        """
+        super(OnDemandCredentials, self).__init__()
+        self._signer = signer
+        self._issuer = issuer
+        self._subject = subject
+        self._token_lifetime = token_lifetime
+
+        if additional_claims is None:
+            additional_claims = {}
+
+        self._additional_claims = additional_claims
+        self._cache = cachetools.LRUCache(maxsize=max_cache_size)
+
+    @classmethod
+    def _from_signer_and_info(cls, signer, info, **kwargs):
+        """Creates an OnDemandCredentials instance from a signer and service
+        account info.
+
+        Args:
+            signer (google.auth.crypt.Signer): The signer used to sign JWTs.
+            info (Mapping[str, str]): The service account info.
+            kwargs: Additional arguments to pass to the constructor.
+
+        Returns:
+            google.auth.jwt.OnDemandCredentials: The constructed credentials.
+
+        Raises:
+            ValueError: If the info is not in the expected format.
+        """
+        kwargs.setdefault('subject', info['client_email'])
+        kwargs.setdefault('issuer', info['client_email'])
+        return cls(signer, **kwargs)
+
+    @classmethod
+    def from_service_account_info(cls, info, **kwargs):
+        """Creates an OnDemandCredentials instance from a dictionary.
+
+        Args:
+            info (Mapping[str, str]): The service account info in Google
+                format.
+            kwargs: Additional arguments to pass to the constructor.
+
+        Returns:
+            google.auth.jwt.OnDemandCredentials: The constructed credentials.
+
+        Raises:
+            ValueError: If the info is not in the expected format.
+        """
+        signer = _service_account_info.from_dict(
+            info, require=['client_email'])
+        return cls._from_signer_and_info(signer, info, **kwargs)
+
+    @classmethod
+    def from_service_account_file(cls, filename, **kwargs):
+        """Creates an OnDemandCredentials instance from a service account .json
+        file in Google format.
+
+        Args:
+            filename (str): The path to the service account .json file.
+            kwargs: Additional arguments to pass to the constructor.
+
+        Returns:
+            google.auth.jwt.OnDemandCredentials: The constructed credentials.
+        """
+        info, signer = _service_account_info.from_filename(
+            filename, require=['client_email'])
+        return cls._from_signer_and_info(signer, info, **kwargs)
+
+    @classmethod
+    def from_signing_credentials(cls, credentials, **kwargs):
+        """Creates a new :class:`google.auth.jwt.OnDemandCredentials` instance
+        from an existing :class:`google.auth.credentials.Signing` instance.
+
+        The new instance will use the same signer as the existing instance and
+        will use the existing instance's signer email as the issuer and
+        subject by default.
+
+        Example::
+
+            svc_creds = service_account.Credentials.from_service_account_file(
+                'service_account.json')
+            jwt_creds = jwt.OnDemandCredentials.from_signing_credentials(
+                svc_creds)
+
+        Args:
+            credentials (google.auth.credentials.Signing): The credentials to
+                use to construct the new credentials.
+            kwargs: Additional arguments to pass to the constructor.
+
+        Returns:
+            google.auth.jwt.Credentials: A new Credentials instance.
+        """
+        kwargs.setdefault('issuer', credentials.signer_email)
+        kwargs.setdefault('subject', credentials.signer_email)
+        return cls(credentials.signer, **kwargs)
+
+    def with_claims(self, issuer=None, subject=None, additional_claims=None):
+        """Returns a copy of these credentials with modified claims.
+
+        Args:
+            issuer (str): The `iss` claim. If unspecified the current issuer
+                claim will be used.
+            subject (str): The `sub` claim. If unspecified the current subject
+                claim will be used.
+            additional_claims (Mapping[str, str]): Any additional claims for
+                the JWT payload. This will be merged with the current
+                additional claims.
+
+        Returns:
+            google.auth.jwt.OnDemandCredentials: A new credentials instance.
+        """
+        new_additional_claims = copy.deepcopy(self._additional_claims)
+        new_additional_claims.update(additional_claims or {})
+
+        return OnDemandCredentials(
+            self._signer,
+            issuer=issuer if issuer is not None else self._issuer,
+            subject=subject if subject is not None else self._subject,
+            additional_claims=new_additional_claims,
+            max_cache_size=self._cache.maxsize)
+
+    @property
+    def valid(self):
+        """Checks the validity of the credentials.
+
+        These credentials are always valid because it generates tokens on
+        demand.
+        """
+        return True
+
+    def _make_jwt_for_audience(self, audience):
+        """Make a new JWT for the given audience.
+
+        Args:
+            audience (str): The intended audience.
+
+        Returns:
+            Tuple[bytes, datetime]: The encoded JWT and the expiration.
+        """
+        now = _helpers.utcnow()
+        lifetime = datetime.timedelta(seconds=self._token_lifetime)
+        expiry = now + lifetime
+
+        payload = {
+            'iss': self._issuer,
+            'sub': self._subject,
+            'iat': _helpers.datetime_to_secs(now),
+            'exp': _helpers.datetime_to_secs(expiry),
+            'aud': audience,
+        }
+
+        payload.update(self._additional_claims)
+
+        jwt = encode(self._signer, payload)
+
+        return jwt, expiry
+
+    def _get_jwt_for_audience(self, audience):
+        """Get a JWT For a given audience.
+
+        If there is already an existing, non-expired token in the cache for
+        the audience, that token is used. Otherwise, a new token will be
+        created.
+
+        Args:
+            audience (str): The intended audience.
+
+        Returns:
+            bytes: The encoded JWT.
+        """
+        token, expiry = self._cache.get(audience, (None, None))
+
+        if token is None or expiry < _helpers.utcnow():
+            token, expiry = self._make_jwt_for_audience(audience)
+            self._cache[audience] = token, expiry
+
+        return token
+
+    def refresh(self, request):
+        """Raises an exception, these credentials can not be directly
+        refreshed.
+
+        Args:
+            request (Any): Unused.
+
+        Raises:
+            google.auth.RefreshError
+        """
+        # pylint: disable=unused-argument
+        # (pylint doesn't correctly recognize overridden methods.)
+        raise exceptions.RefreshError(
+            'OnDemandCredentials can not be directly refreshed.')
+
+    def before_request(self, request, method, url, headers):
+        """Performs credential-specific before request logic.
+
+        Args:
+            request (Any): Unused. JWT credentials do not need to make an
+                HTTP request to refresh.
+            method (str): The request's HTTP method.
+            url (str): The request's URI. This is used as the audience claim
+                when generating the JWT.
+            headers (Mapping): The request's headers.
+        """
+        # pylint: disable=unused-argument
+        # (pylint doesn't correctly recognize overridden methods.)
+        parts = urllib.parse.urlsplit(url)
+        # Strip query string and fragment
+        audience = urllib.parse.urlunsplit(
+            (parts.scheme, parts.netloc, parts.path, None, None))
+        token = self._get_jwt_for_audience(audience)
+        self.apply(headers, token=token)
+
+    @_helpers.copy_docstring(google.auth.credentials.Signing)
+    def sign_bytes(self, message):
+        return self._signer.sign(message)
+
+    @property
+    @_helpers.copy_docstring(google.auth.credentials.Signing)
+    def signer_email(self):
+        return self._issuer
+
+    @property
+    @_helpers.copy_docstring(google.auth.credentials.Signing)
+    def signer(self):
+        return self._signer