diff --git a/HowToWriteTests.md b/HowToWriteTests.md
index f334d95..fe0bfab 100644
--- a/HowToWriteTests.md
+++ b/HowToWriteTests.md
@@ -27,6 +27,13 @@ class T(TestCase):
 
 Now run it!
 
+You may notice some existing tests use the `mock` module.
+We are phasing out this style of testing.
+Please do not use it in new tests.
+Instead, factor units and interfaces so mocks are not required.
+If that is not possible then implement units so that they don't need to be mocked and can be used in the test suite.
+If that is not possible then implement a verified fake of the required interface and use that in the test suite.
+
 # code coverage
 
 Now install Ned Batchelder's "[coverage"](http://nedbatchelder.com/code/coverage/) tool and run your test with code coverage, like this: