diff --git a/src/FactorGraphs/FactorList.php b/src/FactorGraphs/FactorList.php
index 2a9a494..2f9585a 100644
--- a/src/FactorGraphs/FactorList.php
+++ b/src/FactorGraphs/FactorList.php
@@ -1,9 +1,11 @@
 <?php namespace Moserware\Skills\FactorGraphs;
 
+use Countable;
+
 /**
  * Helper class for computing the factor graph's normalization constant.
  */
-class FactorList
+class FactorList implements Countable
 {
     private $_list = array();
 
@@ -47,4 +49,4 @@ public function addFactor(Factor $factor)
         $this->_list[] = $factor;
         return $factor;
     }
-}
\ No newline at end of file
+}
diff --git a/src/HashMap.php b/src/HashMap.php
index a3f6971..9ae36fd 100644
--- a/src/HashMap.php
+++ b/src/HashMap.php
@@ -1,9 +1,11 @@
 <?php namespace Moserware\Skills;
 
+use Countable;
+
 /**
  * Basic hashmap that supports object keys.
  */
-class HashMap
+class HashMap implements Countable
 {
     private $_hashToValue = array();
     private $_hashToKey = array();
@@ -48,4 +50,4 @@ private static function getHash($key)
 
         return $key;
     }
-}
\ No newline at end of file
+}
diff --git a/src/RatingContainer.php b/src/RatingContainer.php
index a18cc26..86dbc9e 100644
--- a/src/RatingContainer.php
+++ b/src/RatingContainer.php
@@ -1,6 +1,8 @@
 <?php namespace Moserware\Skills;
 
-class RatingContainer
+use Countable;
+
+class RatingContainer implements Countable
 {
     private $_playerToRating;