diff --git a/.travis.yml b/.travis.yml
index 5e3cfe56..58af5e26 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -6,10 +6,12 @@ python:
 - '3.5'
 - '3.6'
 install:
+- pip install mypy
 - pip install coveralls
 - pip install pycodestyle
 - pip install --upgrade .
 script:
+- mypy --ignore-missing-imports houston
 - pycodestyle houston
 - pytest test
 - coverage run --source=houston setup.py test
diff --git a/houston/predicate.py b/houston/predicate.py
index 575a8ea7..84c789b0 100644
--- a/houston/predicate.py
+++ b/houston/predicate.py
@@ -1,9 +1,13 @@
-__all__ = ['Predicate']
+__all__ = ['Predicate', 'Invariant', 'Postcondition', 'Precondition']
 
 from typing import Callable
 
 import attr
 
+from .action import Action
+from .state import State
+from .environment import Environment
+
 
 @attr.s(frozen=True)
 class Predicate(object):
@@ -13,13 +17,13 @@ class Predicate(object):
     of purposes.
     """
     name = attr.ib(type=str)
-    predicate = attr.ib(type=Callable[['Action', 'State', 'Environment'],
+    predicate = attr.ib(type=Callable[[Action, State, Environment],
                                       bool])
 
     def check(self,
-              action: 'Action',
-              state: 'State',
-              environment: 'Environment'
+              action: Action,
+              state: State,
+              environment: Environment
               ) -> bool:
         """
         Determines whether this predicate is satisfied by a given action,