diff --git a/django/core/cache/backends/base.py b/django/core/cache/backends/base.py
--- a/django/core/cache/backends/base.py
+++ b/django/core/cache/backends/base.py
@@ -5,6 +5,12 @@
 class InvalidCacheBackendError(ImproperlyConfigured):
     pass
 
+class InvalidCacheKeyError(Exception):
+    pass
+
+# memcached does not accept keys longer than 250 characters
+MAX_KEY_LENGTH = 250
+
 class BaseCache(object):
     def __init__(self, params):
         timeout = params.get('timeout', 300)
@@ -116,3 +122,15 @@
     def clear(self):
         """Remove *all* values from the cache at once."""
         raise NotImplementedError
+
+    def _validate_key(self, key):
+        """
+        Enforce keys that would be valid on all backends, to keep cache code portable.
+
+        """
+        if len(key) > MAX_KEY_LENGTH:
+            raise InvalidCacheKeyError('Key is too long (max length %s)' % MAX_KEY_LENGTH)
+        for char in key:
+            if ord(char) < 33 or ord(char) == 127:
+                raise InvalidCacheKeyError('Key contains control character')
+        
diff --git a/django/core/cache/backends/db.py b/django/core/cache/backends/db.py
--- a/django/core/cache/backends/db.py
+++ b/django/core/cache/backends/db.py
@@ -46,6 +46,7 @@
             self._cull_frequency = 3
 
     def get(self, key, default=None):
+        self._validate_key(key)
         db = router.db_for_read(self.cache_model_class)
         table = connections[db].ops.quote_name(self._table)
         cursor = connections[db].cursor()
@@ -65,9 +66,11 @@
         return pickle.loads(base64.decodestring(value))
 
     def set(self, key, value, timeout=None):
+        self._validate_key(key)
         self._base_set('set', key, value, timeout)
 
     def add(self, key, value, timeout=None):
+        self._validate_key(key)
         return self._base_set('add', key, value, timeout)
 
     def _base_set(self, mode, key, value, timeout=None):
@@ -103,6 +106,7 @@
             return True
 
     def delete(self, key):
+        self._validate_key(key)
         db = router.db_for_write(self.cache_model_class)
         table = connections[db].ops.quote_name(self._table)
         cursor = connections[db].cursor()
@@ -111,6 +115,7 @@
         transaction.commit_unless_managed(using=db)
 
     def has_key(self, key):
+        self._validate_key(key)
         db = router.db_for_read(self.cache_model_class)
         table = connections[db].ops.quote_name(self._table)
         cursor = connections[db].cursor()
diff --git a/django/core/cache/backends/dummy.py b/django/core/cache/backends/dummy.py
--- a/django/core/cache/backends/dummy.py
+++ b/django/core/cache/backends/dummy.py
@@ -6,22 +6,25 @@
     def __init__(self, *args, **kwargs):
         pass
 
-    def add(self, *args, **kwargs):
+    def add(self, key, *args, **kwargs):
+        self._validate_key(key)
         return True
 
     def get(self, key, default=None):
+        self._validate_key(key)
         return default
 
-    def set(self, *args, **kwargs):
-        pass
+    def set(self, key, *args, **kwargs):
+        self._validate_key(key)
 
-    def delete(self, *args, **kwargs):
-        pass
+    def delete(self, key, *args, **kwargs):
+        self._validate_key(key)
 
     def get_many(self, *args, **kwargs):
         return {}
 
-    def has_key(self, *args, **kwargs):
+    def has_key(self, key, *args, **kwargs):
+        self._validate_key(key)
         return False
 
     def set_many(self, *args, **kwargs):
diff --git a/django/core/cache/backends/filebased.py b/django/core/cache/backends/filebased.py
--- a/django/core/cache/backends/filebased.py
+++ b/django/core/cache/backends/filebased.py
@@ -32,6 +32,7 @@
             self._createdir()
 
     def add(self, key, value, timeout=None):
+        self._validate_key(key)
         if self.has_key(key):
             return False
 
@@ -39,6 +40,7 @@
         return True
 
     def get(self, key, default=None):
+        self._validate_key(key)
         fname = self._key_to_file(key)
         try:
             f = open(fname, 'rb')
@@ -56,6 +58,7 @@
         return default
 
     def set(self, key, value, timeout=None):
+        self._validate_key(key)
         fname = self._key_to_file(key)
         dirname = os.path.dirname(fname)
 
@@ -79,6 +82,7 @@
             pass
 
     def delete(self, key):
+        self._validate_key(key)
         try:
             self._delete(self._key_to_file(key))
         except (IOError, OSError):
@@ -95,6 +99,7 @@
             pass
 
     def has_key(self, key):
+        self._validate_key(key)
         fname = self._key_to_file(key)
         try:
             f = open(fname, 'rb')
diff --git a/django/core/cache/backends/locmem.py b/django/core/cache/backends/locmem.py
--- a/django/core/cache/backends/locmem.py
+++ b/django/core/cache/backends/locmem.py
@@ -30,6 +30,7 @@
         self._lock = RWLock()
 
     def add(self, key, value, timeout=None):
+        self._validate_key(key)
         self._lock.writer_enters()
         try:
             exp = self._expire_info.get(key)
@@ -44,6 +45,7 @@
             self._lock.writer_leaves()
 
     def get(self, key, default=None):
+        self._validate_key(key)
         self._lock.reader_enters()
         try:
             exp = self._expire_info.get(key)
@@ -76,6 +78,7 @@
         self._expire_info[key] = time.time() + timeout
 
     def set(self, key, value, timeout=None):
+        self._validate_key(key)
         self._lock.writer_enters()
         # Python 2.4 doesn't allow combined try-except-finally blocks.
         try:
@@ -87,6 +90,7 @@
             self._lock.writer_leaves()
 
     def has_key(self, key):
+        self._validate_key(key)
         self._lock.reader_enters()
         try:
             exp = self._expire_info.get(key)
@@ -127,6 +131,7 @@
             pass
 
     def delete(self, key):
+        self._validate_key(key)
         self._lock.writer_enters()
         try:
             self._delete(key)
diff --git a/docs/topics/cache.txt b/docs/topics/cache.txt
--- a/docs/topics/cache.txt
+++ b/docs/topics/cache.txt
@@ -537,6 +537,11 @@
     >>> cache.get('my_key')
     'hello, world!'
 
+.. note::
+
+   Cache keys may not be longer than 250 characters, and may not
+   contain whitespace or control characters.
+
 The ``timeout`` argument is optional and defaults to the ``timeout``
 argument in the ``CACHE_BACKEND`` setting (explained above). It's the number of
 seconds the value should be stored in the cache.
diff --git a/tests/regressiontests/cache/tests.py b/tests/regressiontests/cache/tests.py
--- a/tests/regressiontests/cache/tests.py
+++ b/tests/regressiontests/cache/tests.py
@@ -366,6 +366,21 @@
                 count = count + 1
         self.assertEqual(count, final_count)
 
+    def test_invalid_keys(self):
+        """
+        All the builtin backends should refuse keys that would be refused by
+        memcached, so code using the cache API is portable. Refs #6447.
+
+        We assert only that a generic exception is raised because we don't
+        want to encumber the memcached backend with key-checking code, and the
+        specific exceptions raised from memcached will depend on the client
+        library used.
+        
+        """
+        self.assertRaises(Exception, self.cache.set, 'key with spaces', 'value')
+        # memcached also limits key length to 250
+        self.assertRaises(Exception, self.cache.set, 'a' * 251, 'value')
+
 class DBCacheTests(unittest.TestCase, BaseCacheTests):
     def setUp(self):
         # Spaces are used in the table name to ensure quoting/escaping is working