diff --git a/CHANGES b/CHANGES
index 0eb34dc1..1dca6ee6 100644
--- a/CHANGES
+++ b/CHANGES
@@ -18,6 +18,7 @@ Release date to be announced, codename to be selected.
   requests that do not pop the request stack for testing.
 - because the Python standard library caches loggers, the name of
   the logger is configurable now to better support unittests.
+- added `TESTING` switch that can activate unittesting helpers.
 
 Version 0.3.1
 -------------
diff --git a/docs/config.rst b/docs/config.rst
index de79cabc..e057ba4d 100644
--- a/docs/config.rst
+++ b/docs/config.rst
@@ -53,6 +53,7 @@ The following configuration values are used internally by Flask:
 
 =============================== =========================================
 ``DEBUG``                       enable/disable debug mode
+``TESTING``                     enable/disable testing mode
 ``SECRET_KEY``                  the secret key
 ``SESSION_COOKIE_NAME``         the name of the session cookie
 ``PERMANENT_SESSION_LIFETIME``  the lifetime of a permanent session as
diff --git a/flask.py b/flask.py
index ca69a056..c4656cdb 100644
--- a/flask.py
+++ b/flask.py
@@ -810,6 +810,15 @@ class Flask(_PackageBoundObject):
     #: configuration key.  Defaults to `False`.
     debug = ConfigAttribute('DEBUG')
 
+    #: The testing flask.  Set this to `True` to enable the test mode of
+    #: Flask extensions (and in the future probably also Flask itself).
+    #: For example this might activate unittest helpers that have an
+    #: additional runtime cost which should not be enabled by default.
+    #:
+    #: This attribute can also be configured from the config with the
+    #: `TESTING` configuration key.  Defaults to `False`.
+    testing = ConfigAttribute('TESTING')
+
     #: If a secret key is set, cryptographic components can use this to
     #: sign cookies and other things.  Set this to a complex random value
     #: when you want to use the secure cookie for instance.
@@ -870,6 +879,7 @@ class Flask(_PackageBoundObject):
     #: Default configuration parameters.
     default_config = ImmutableDict({
         'DEBUG':                                False,
+        'TESTING':                              False,
         'SECRET_KEY':                           None,
         'SESSION_COOKIE_NAME':                  'session',
         'PERMANENT_SESSION_LIFETIME':           timedelta(days=31),