diff --git a/NEWS b/NEWS
index 2d8037bb..94605b0c 100644
--- a/NEWS
+++ b/NEWS
@@ -17,6 +17,7 @@ NEWS ( CHANGELOG and HISTORY )                                     HTMLPurifier
   + Constructor methods renamed to __construct()
   + PHP4 reference/foreach cruft removed (in progress)
 ! CSS properties are no case-insensitive
+! DefinitionCacheFactory now can register new implementations
 . Unit tests for Injector improved
 
 2.1.3, released 2007-11-05
diff --git a/library/HTMLPurifier/DefinitionCache.php b/library/HTMLPurifier/DefinitionCache.php
index 88a5abb9..93f149ef 100644
--- a/library/HTMLPurifier/DefinitionCache.php
+++ b/library/HTMLPurifier/DefinitionCache.php
@@ -104,6 +104,9 @@ abstract class HTMLPurifier_DefinitionCache
     
     /**
      * Clears all expired (older version or revision) objects from cache
+     * @note Be carefuly implementing this method as flush. Flush must
+     *       not interfere with other Definition types, and cleanup()
+     *       should not be repeatedly called by userland code.
      */
     abstract public function cleanup($config);
     
diff --git a/library/HTMLPurifier/DefinitionCacheFactory.php b/library/HTMLPurifier/DefinitionCacheFactory.php
index 4fa0ab5f..cdbeb1d4 100644
--- a/library/HTMLPurifier/DefinitionCacheFactory.php
+++ b/library/HTMLPurifier/DefinitionCacheFactory.php
@@ -10,10 +10,6 @@ to disable caching (not recommended, as you will see a definite
 performance degradation). This directive has been available since 2.0.0.
 ');
 
-HTMLPurifier_ConfigSchema::defineAllowedValues(
-    'Cache', 'DefinitionImpl', array('Serializer')
-);
-
 HTMLPurifier_ConfigSchema::defineAlias(
     'Core', 'DefinitionCache',
     'Cache', 'DefinitionImpl'
@@ -27,6 +23,7 @@ class HTMLPurifier_DefinitionCacheFactory
 {
     
     protected $caches = array('Serializer' => array());
+    protected $implementations = array();
     protected $decorators = array();
     
     /**
@@ -50,14 +47,21 @@ class HTMLPurifier_DefinitionCacheFactory
         return $instance;
     }
     
+    /**
+     * Registers a new definition cache object
+     * @param $short Short name of cache object, for reference
+     * @param $long Full class name of cache object, for construction 
+     */
+    public function register($short, $long) {
+        $this->implementations[$short] = $long;
+    }
+    
     /**
      * Factory method that creates a cache object based on configuration
      * @param $name Name of definitions handled by cache
      * @param $config Instance of HTMLPurifier_Config
      */
     public function &create($type, $config) {
-        // only one implementation as for right now, $config will
-        // be used to determine implementation
         $method = $config->get('Cache', 'DefinitionImpl');
         if ($method === null) {
             $null = new HTMLPurifier_DefinitionCache_Null($type);
@@ -66,7 +70,17 @@ class HTMLPurifier_DefinitionCacheFactory
         if (!empty($this->caches[$method][$type])) {
             return $this->caches[$method][$type];
         }
-        $cache = new HTMLPurifier_DefinitionCache_Serializer($type);
+        if (
+          isset($this->implementations[$method]) &&
+          class_exists($class = $this->implementations[$method], false)
+        ) {
+            $cache = new $class($type);
+        } else {
+            if ($method != 'Serializer') {
+                trigger_error("Unrecognized DefinitionCache $method, using Serializer instead", E_USER_WARNING);
+            }
+            $cache = new HTMLPurifier_DefinitionCache_Serializer($type);
+        }
         foreach ($this->decorators as $decorator) {
             $new_cache = $decorator->decorate($cache);
             // prevent infinite recursion in PHP 4
diff --git a/tests/HTMLPurifier/DefinitionCacheFactoryTest.php b/tests/HTMLPurifier/DefinitionCacheFactoryTest.php
index ba81801a..d2585be1 100644
--- a/tests/HTMLPurifier/DefinitionCacheFactoryTest.php
+++ b/tests/HTMLPurifier/DefinitionCacheFactoryTest.php
@@ -5,13 +5,14 @@ require_once 'HTMLPurifier/DefinitionCacheFactory.php';
 class HTMLPurifier_DefinitionCacheFactoryTest extends HTMLPurifier_Harness
 {
     
-    protected $newFactory;
+    protected $factory;
     protected $oldFactory;
     
     function setup() {
-        $new = new HTMLPurifier_DefinitionCacheFactory();
+        parent::setup();
+        $this->factory = new HTMLPurifier_DefinitionCacheFactory();
         $this->oldFactory = HTMLPurifier_DefinitionCacheFactory::instance();
-        HTMLPurifier_DefinitionCacheFactory::instance($new);
+        HTMLPurifier_DefinitionCacheFactory::instance($this->factory);
     }
     
     function teardown() {
@@ -19,46 +20,52 @@ class HTMLPurifier_DefinitionCacheFactoryTest extends HTMLPurifier_Harness
     }
     
     function test_create() {
-        $config  = HTMLPurifier_Config::createDefault();
-        $factory = HTMLPurifier_DefinitionCacheFactory::instance();
-        $cache   = $factory->create('Test', $config);
+        $cache = $this->factory->create('Test', $this->config);
         $this->assertEqual($cache, new HTMLPurifier_DefinitionCache_Serializer('Test'));
     }
     
     function test_create_withDecorator() {
-        $config  = HTMLPurifier_Config::createDefault();
-        $factory =& HTMLPurifier_DefinitionCacheFactory::instance();
-        $factory->addDecorator('Memory');
-        $cache =& $factory->create('Test', $config);
+        $this->factory->addDecorator('Memory');
+        $cache = $this->factory->create('Test', $this->config);
         $cache_real = new HTMLPurifier_DefinitionCache_Decorator_Memory();
         $cache_real = $cache_real->decorate(new HTMLPurifier_DefinitionCache_Serializer('Test'));
         $this->assertEqual($cache, $cache_real);
     }
     
     function test_create_withDecoratorObject() {
-        $config  = HTMLPurifier_Config::createDefault();
-        $factory =& HTMLPurifier_DefinitionCacheFactory::instance();
-        $factory->addDecorator(new HTMLPurifier_DefinitionCache_Decorator_Memory());
-        $cache =& $factory->create('Test', $config);
+        $this->factory->addDecorator(new HTMLPurifier_DefinitionCache_Decorator_Memory());
+        $cache = $this->factory->create('Test', $this->config);
         $cache_real = new HTMLPurifier_DefinitionCache_Decorator_Memory();
         $cache_real = $cache_real->decorate(new HTMLPurifier_DefinitionCache_Serializer('Test'));
         $this->assertEqual($cache, $cache_real);
     }
     
     function test_create_recycling() {
-        $config  = HTMLPurifier_Config::createDefault();
-        $factory =& HTMLPurifier_DefinitionCacheFactory::instance();
-        $cache =& $factory->create('Test', $config);
-        $cache2 =& $factory->create('Test', $config);
+        $cache  = $this->factory->create('Test', $this->config);
+        $cache2 = $this->factory->create('Test', $this->config);
         $this->assertReference($cache, $cache2);
     }
     
+    function test_create_invalid() {
+        $this->config->set('Core', 'DefinitionCache', 'Invalid');
+        $this->expectError('Unrecognized DefinitionCache Invalid, using Serializer instead');
+        $cache = $this->factory->create('Test', $this->config);
+        $this->assertIsA($cache, 'HTMLPurifier_DefinitionCache_Serializer');
+    }
+    
     function test_null() {
-        $config = HTMLPurifier_Config::create(array('Core.DefinitionCache' => null));
-        $factory =& HTMLPurifier_DefinitionCacheFactory::instance();
-        $cache =& $factory->create('Test', $config);
+        $this->config->set('Core', 'DefinitionCache', null);
+        $cache = $this->factory->create('Test', $this->config);
         $this->assertEqual($cache, new HTMLPurifier_DefinitionCache_Null('Test'));
     }
     
+    function test_register() {
+        generate_mock_once('HTMLPurifier_DefinitionCache');
+        $this->config->set('Core', 'DefinitionCache', 'TestCache');
+        $this->factory->register('TestCache', $class = 'HTMLPurifier_DefinitionCacheMock');
+        $cache = $this->factory->create('Test', $this->config);
+        $this->assertIsA($cache, $class);
+    }
+    
 }