diff --git a/README.md b/README.md
index 39482b0..90f776e 100644
--- a/README.md
+++ b/README.md
@@ -36,10 +36,21 @@
 
 ## Module Control.Monad
 
+### Type Classes
+
+    class (MonadZero m) <= MonadPlus m where
+      mplus :: forall a. m a -> m a -> m a
+
+    class (Monad m) <= MonadZero m where
+      mzero :: forall a. m a
+
+
 ### Values
 
     foldM :: forall m a b. (Monad m) => (a -> b -> m a) -> a -> [b] -> m a
 
+    guard :: forall m. (MonadPlus m) => Prim.Boolean -> m Unit
+
     replicateM :: forall m a. (Monad m) => Prim.Number -> m a -> m [a]
 
     unless :: forall m. (Monad m) => Prim.Boolean -> m {  } -> m {  }
diff --git a/src/Control/Monad.purs b/src/Control/Monad.purs
index e7b6e4c..4ec415a 100644
--- a/src/Control/Monad.purs
+++ b/src/Control/Monad.purs
@@ -1,5 +1,11 @@
 module Control.Monad where
 
+class (Monad m) <= MonadZero m where
+  mzero :: forall a. m a
+
+class (MonadZero m) <= MonadPlus m where
+  mplus :: forall a. m a -> m a -> m a
+
 replicateM :: forall m a. (Monad m) => Number -> m a -> m [a]
 replicateM 0 _ = return []
 replicateM n m = do
@@ -11,6 +17,10 @@ foldM :: forall m a b. (Monad m) => (a -> b -> m a) -> a -> [b] -> m a
 foldM _ a [] = return a
 foldM f a (b:bs) = f a b >>= \a' -> foldM f a' bs
 
+guard :: forall m. (MonadPlus m) => Boolean -> m Unit
+guard true = return unit
+guard false = mzero
+
 when :: forall m. (Monad m) => Boolean -> m {} -> m {}
 when true m = m
 when false _ = return {}