diff --git a/starlette/authentication.py b/starlette/authentication.py
index f7c221e5..9aa4ca5e 100644
--- a/starlette/authentication.py
+++ b/starlette/authentication.py
@@ -1,5 +1,6 @@
 import asyncio
 import functools
+import inspect
 import typing
 
 from starlette.exceptions import HTTPException
@@ -22,35 +23,39 @@ def requires(
     scopes_list = [scopes] if isinstance(scopes, str) else list(scopes)
 
     def decorator(func: typing.Callable) -> typing.Callable:
+        sig = inspect.signature(func)
+        for idx, parameter in enumerate(sig.parameters.values()):
+            if parameter.name == "request":
+                break
+        else:
+            raise Exception('No "request" argument on function "%s"' % func)
+
         if asyncio.iscoroutinefunction(func):
 
             @functools.wraps(func)
-            async def wrapper(*args: typing.Any) -> Response:
-                # Support either `func(request)`` or `func(self, request)``
-                assert len(args) in (1, 2)
-                request = args[-1]
+            async def wrapper(*args: typing.Any, **kwargs: typing.Any) -> Response:
+                request = kwargs.get("request", args[idx])
                 assert isinstance(request, Request)
 
                 if not has_required_scope(request, scopes_list):
                     if redirect is not None:
                         return RedirectResponse(url=request.url_for(redirect))
                     raise HTTPException(status_code=status_code)
-                return await func(*args)
+                return await func(*args, **kwargs)
 
             return wrapper
 
         @functools.wraps(func)
-        def sync_wrapper(*args: typing.Any) -> Response:
-            # Support either `func(request)`` or `func(self, request)``
-            assert len(args) in (1, 2)
-            request = args[-1]
+        def sync_wrapper(*args: typing.Any, **kwargs: typing.Any) -> Response:
+            # Support either `func(request)` or `func(self, request)`
+            request = kwargs.get("request", args[idx])
             assert isinstance(request, Request)
 
             if not has_required_scope(request, scopes_list):
                 if redirect is not None:
                     return RedirectResponse(url=request.url_for(redirect))
                 raise HTTPException(status_code=status_code)
-            return func(*args)
+            return func(*args, **kwargs)
 
         return sync_wrapper
 
diff --git a/tests/test_authentication.py b/tests/test_authentication.py
index 829d9802..038a0978 100644
--- a/tests/test_authentication.py
+++ b/tests/test_authentication.py
@@ -1,6 +1,8 @@
 import base64
 import binascii
 
+import pytest
+
 from starlette.applications import Starlette
 from starlette.authentication import (
     AuthCredentials,
@@ -102,6 +104,14 @@ def admin(request):
     )
 
 
+def test_invalid_decorator_usage():
+    with pytest.raises(Exception):
+
+        @requires("authenticated")
+        def foo():
+            pass  # pragma: nocover
+
+
 def test_user_interface():
     with TestClient(app) as client:
         response = client.get("/")