diff --git a/runtests.sh b/runtests.sh
index 07bd360a..4058c024 100755
--- a/runtests.sh
+++ b/runtests.sh
@@ -1,4 +1,12 @@
 #!/bin/sh
+# Run the Tornado test suite.
+#
+# Also consider using tox, which uses virtualenv to run the test suite
+# under multiple versions of python.
+#
+# This script requires that `python` is python 2.x; to run the tests under
+# python 3 tornado must be installed so that 2to3 is run.  The easiest
+# way to run the tests under python 3 is with tox: "tox -e py32".
 
 cd $(dirname $0)