diff --git a/requirements.txt b/requirements.txt
index c2ab3ed..1b5f83c 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,9 +1,9 @@
-tornado==5.1
-pyzmq==17.1.2
-transitions==0.6.6
-terminado==0.8.1
-watchdog==0.8.3
-PyYAML==3.13
-Jinja2==2.10
-cryptography==2.3.1
-python-dateutil==2.7.3
+tornado>=6.0.0,<7.0.0
+pyzmq>=17.0.0,<18.0.0
+transitions>=0.0.0,<1.0.0
+terminado>=0.0.0,<1.0.0
+watchdog>=0.0.0,<1.0.0
+PyYAML>=5.0.0,<6.0.0
+Jinja2>=2.0.0,<3.0.0
+cryptography>=2.0.0,<3.0.0
+python-dateutil>=2.0.0,<3.0.0