diff --git a/docs/contributing.rst b/docs/contributing.rst
--- a/docs/contributing.rst
+++ b/docs/contributing.rst
@@ -48,11 +48,15 @@ on Bitbucket_ and create a local clone o
 Running tests
 -------------
 
-After finishing your changes make sure all tests pass cleanly. You can run
-the testsuite by running ``py.test`` from the project root::
+After finishing your changes make sure all tests pass cleanly. Install the test
+dependencies, then run the testsuite by invoking ``py.test`` from the
+project root::
 
+    pip install -r dev_requirements.txt
     py.test
 
+Note that testing on Python 2.6 also requires ``unittest2``.
+
 You can also use ``tox`` to run the tests with all supported Python versions
 (currently Python 2.6--2.7).