diff --git a/docs/history.rst b/docs/history.rst
index 665c54a..0e66ee7 100644
--- a/docs/history.rst
+++ b/docs/history.rst
@@ -6,6 +6,7 @@ Release History
 `Next Release`_
 ---------------
 - Add configuration documentation
+- Exclude Tornado >6 (as-yet-unreleased version)
 
 `4.0.0`_ (06-Feb-2019)
 ----------------------
diff --git a/requires/installation.txt b/requires/installation.txt
index 293b5b2..616640f 100644
--- a/requires/installation.txt
+++ b/requires/installation.txt
@@ -1 +1 @@
-tornado>=5
+tornado>=5,<7