diff --git a/doc/testing.md b/doc/testing.md
index 89ddda9c34b..fb8149f8db1 100644
--- a/doc/testing.md
+++ b/doc/testing.md
@@ -1,5 +1,16 @@
 # Testing
 
+## External dependencies
+
+Some of the smart-answers tests require PhantomJS to be [installed on your machine
+natively](https://github.com/teampoltergeist/poltergeist/blob/master/README.md#installing-phantomjs).
+
+Smart-answers also require the govuk-content-schemas repository which can
+be [cloned](https://github.com/alphagov/govuk-content-schemas) into a sibling
+directory, or a directory referenced using GOVUK_CONTENT_SCHEMAS_PATH.
+
+## Executing tests
+
 Run all tests by executing the following:
 
     bundle exec rake