diff --git a/httpx/__init__.py b/httpx/__init__.py
index af38f8a912..19ab42b5c3 100644
--- a/httpx/__init__.py
+++ b/httpx/__init__.py
@@ -1,7 +1,7 @@
 from .__version__ import __description__, __title__, __version__
 from ._api import delete, get, head, options, patch, post, put, request, stream
 from ._auth import Auth, BasicAuth, DigestAuth
-from ._client import AsyncClient, Client
+from ._client import USE_CLIENT_DEFAULT, AsyncClient, Client
 from ._config import Limits, Proxy, Timeout, create_ssl_context
 from ._content import ByteStream
 from ._exceptions import (
@@ -112,6 +112,7 @@
     "TransportError",
     "UnsupportedProtocol",
     "URL",
+    "USE_CLIENT_DEFAULT",
     "WriteError",
     "WriteTimeout",
     "WSGITransport",
diff --git a/httpx/_client.py b/httpx/_client.py
index 429382fa80..e33597f531 100644
--- a/httpx/_client.py
+++ b/httpx/_client.py
@@ -11,6 +11,8 @@
     DEFAULT_MAX_REDIRECTS,
     DEFAULT_TIMEOUT_CONFIG,
     UNSET,
+    USE_CLIENT_DEFAULT,
+    ClientDefaultType,
     Limits,
     Proxy,
     Timeout,
@@ -301,7 +303,7 @@ def stream(
         params: QueryParamTypes = None,
         headers: HeaderTypes = None,
         cookies: CookieTypes = None,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> "StreamContextManager":
@@ -446,9 +448,15 @@ def _build_auth(self, auth: AuthTypes) -> typing.Optional[Auth]:
             raise TypeError(f'Invalid "auth" argument: {auth!r}')
 
     def _build_request_auth(
-        self, request: Request, auth: typing.Union[AuthTypes, UnsetType] = UNSET
+        self,
+        request: Request,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
     ) -> Auth:
-        auth = self._auth if isinstance(auth, UnsetType) else self._build_auth(auth)
+        auth = (
+            self._auth
+            if isinstance(auth, ClientDefaultType)
+            else self._build_auth(auth)
+        )
 
         if auth is not None:
             return auth
@@ -758,7 +766,7 @@ def request(
         params: QueryParamTypes = None,
         headers: HeaderTypes = None,
         cookies: CookieTypes = None,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> Response:
@@ -798,7 +806,7 @@ def send(
         request: Request,
         *,
         stream: bool = False,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> Response:
@@ -958,7 +966,7 @@ def get(
         params: QueryParamTypes = None,
         headers: HeaderTypes = None,
         cookies: CookieTypes = None,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> Response:
@@ -985,7 +993,7 @@ def options(
         params: QueryParamTypes = None,
         headers: HeaderTypes = None,
         cookies: CookieTypes = None,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> Response:
@@ -1012,7 +1020,7 @@ def head(
         params: QueryParamTypes = None,
         headers: HeaderTypes = None,
         cookies: CookieTypes = None,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> Response:
@@ -1043,7 +1051,7 @@ def post(
         params: QueryParamTypes = None,
         headers: HeaderTypes = None,
         cookies: CookieTypes = None,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> Response:
@@ -1078,7 +1086,7 @@ def put(
         params: QueryParamTypes = None,
         headers: HeaderTypes = None,
         cookies: CookieTypes = None,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> Response:
@@ -1113,7 +1121,7 @@ def patch(
         params: QueryParamTypes = None,
         headers: HeaderTypes = None,
         cookies: CookieTypes = None,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> Response:
@@ -1144,7 +1152,7 @@ def delete(
         params: QueryParamTypes = None,
         headers: HeaderTypes = None,
         cookies: CookieTypes = None,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> Response:
@@ -1394,7 +1402,7 @@ async def request(
         params: QueryParamTypes = None,
         headers: HeaderTypes = None,
         cookies: CookieTypes = None,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> Response:
@@ -1435,7 +1443,7 @@ async def send(
         request: Request,
         *,
         stream: bool = False,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> Response:
@@ -1602,7 +1610,7 @@ async def get(
         params: QueryParamTypes = None,
         headers: HeaderTypes = None,
         cookies: CookieTypes = None,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> Response:
@@ -1629,7 +1637,7 @@ async def options(
         params: QueryParamTypes = None,
         headers: HeaderTypes = None,
         cookies: CookieTypes = None,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> Response:
@@ -1656,7 +1664,7 @@ async def head(
         params: QueryParamTypes = None,
         headers: HeaderTypes = None,
         cookies: CookieTypes = None,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> Response:
@@ -1687,7 +1695,7 @@ async def post(
         params: QueryParamTypes = None,
         headers: HeaderTypes = None,
         cookies: CookieTypes = None,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> Response:
@@ -1722,7 +1730,7 @@ async def put(
         params: QueryParamTypes = None,
         headers: HeaderTypes = None,
         cookies: CookieTypes = None,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> Response:
@@ -1757,7 +1765,7 @@ async def patch(
         params: QueryParamTypes = None,
         headers: HeaderTypes = None,
         cookies: CookieTypes = None,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> Response:
@@ -1788,7 +1796,7 @@ async def delete(
         params: QueryParamTypes = None,
         headers: HeaderTypes = None,
         cookies: CookieTypes = None,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
     ) -> Response:
@@ -1877,7 +1885,7 @@ def __init__(
         client: BaseClient,
         request: Request,
         *,
-        auth: typing.Union[AuthTypes, UnsetType] = UNSET,
+        auth: typing.Union[AuthTypes, ClientDefaultType] = USE_CLIENT_DEFAULT,
         allow_redirects: bool = True,
         timeout: typing.Union[TimeoutTypes, UnsetType] = UNSET,
         close_client: bool = False,
diff --git a/httpx/_config.py b/httpx/_config.py
index 837519afb5..858f4478e4 100644
--- a/httpx/_config.py
+++ b/httpx/_config.py
@@ -34,10 +34,15 @@
 
 
 class UnsetType:
+    pass  #  pragma: nocover
+
+
+class ClientDefaultType:
     pass  # pragma: nocover
 
 
 UNSET = UnsetType()
+USE_CLIENT_DEFAULT = ClientDefaultType()
 
 
 def create_ssl_context(