diff --git a/colander/interfaces.py b/colander/interfaces.py
index 82ee077..1048cbe 100644
--- a/colander/interfaces.py
+++ b/colander/interfaces.py
@@ -1,6 +1,19 @@
+def Preparer(value):
+    """
+    A preparer is called after deserialization of a value but before
+    that value is validated.
+    
+    Any modifications to ``value`` required should be made by
+    returning the modified value rather than modifying in-place.
+
+    If no modification is required, then ``value`` should be returned
+    as-is.
+    """
+
+    
 def Validator(node, value):
     """
-    A validator is called after deserialization of a value.
+    A validator is called after preparation of the deserialized value.
     
     If ``value`` is not valid, raise a :class:`colander.Invalid`
     instance as an exception after.
diff --git a/docs/basics.rst b/docs/basics.rst
index ad15ba0..3a0d6bc 100644
--- a/docs/basics.rst
+++ b/docs/basics.rst
@@ -89,15 +89,24 @@ Schema Node Objects
 
 A schema is composed of one or more *schema node* objects, each typically of
 the class :class:`colander.SchemaNode`, usually in a nested arrangement.
-Each schema node object has a required *type*, an optional deserialization
-*validator*, an optional *default*, an optional *missing*, an optional
-*title*, an optional *description*, and a slightly less optional *name*.  It
-also accepts *arbitrary* keyword arguments, which are attached directly as
-attributes to the node instance.
+Each schema node object has a required *type*, an optional *preparer*
+for adjusting data after deserialization, an optional
+*validator* for deserialized prepared data, an optional *default*, an
+optional *missing*, an optional *title*, an optional *description*,
+and a slightly less optional *name*.  It also accepts *arbitrary*
+keyword arguments, which are attached directly as attributes to the
+node instance.
 
 The *type* of a schema node indicates its data type (such as
 :class:`colander.Int` or :class:`colander.String`).
 
+The *preparer* of a schema node is called after
+deserialization but before validation; it prepares a deserialized
+value for validation. Examples would be to prepend schemes that may be
+missing on url values or to filter html provided by a rich text
+editor. A preparer is not called during serialization, only during
+deserialization.
+
 The *validator* of a schema node is called after deserialization; it
 makes sure the deserialized value matches a constraint.  An example of
 such a validator is provided in the schema above:
@@ -392,6 +401,38 @@ which the exception is related.
 See the :class:`colander.Invalid` API documentation for more
 information.
 
+Preparing deserialized data for validation
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In certain circumstances, it is necessary to modify the deserialized
+value before validating it.
+
+For example, a :class:`~colander.String` node may be required to
+contain content, but that content may come from a rich text
+editor. Such an editor may return ``<b></b>`` which may appear to be
+valid but doesn't contain content, or 
+``<a href="javascript:alert('evil'')">good</a>`` which is valid, but
+only after some processing.
+
+The following schema uses `htmllaundry`__ and a
+:class:`~colander.interfaces.Preparer` to do the correct thing in both
+cases:
+
+__ http://pypi.python.org/pypi/htmllaundry/
+
+.. code-block:: python
+   :linenos:
+
+   import colander
+   import htmlaundry
+
+   class Page(colander.MappingSchema):
+       title = colander.SchemaNode(colander.String())
+       content = colander.SchemaNode(colander.String(),
+                                     preparer=htmllaundry.sanitize,
+                                     validator=colander.Length(1))
+
+
 Serialization
 -------------
 
diff --git a/docs/interfaces.rst b/docs/interfaces.rst
index 633070e..563eae9 100644
--- a/docs/interfaces.rst
+++ b/docs/interfaces.rst
@@ -3,6 +3,8 @@ Interfaces
 
 .. automodule:: colander.interfaces
 
+  .. autofunction:: Preparer
+
   .. autofunction:: Validator
 
   .. autoclass:: Type