diff --git a/source/Ox/js/Collection.js b/source/Ox/js/Collection.js
index bfcbbd47..6789272a 100644
--- a/source/Ox/js/Collection.js
+++ b/source/Ox/js/Collection.js
@@ -343,6 +343,25 @@ Ox.min = function(collection) {
     return ret;
 };
 
+/*@
+Ox.numberOf <f> Returns the number of elements in a collection that pass a test
+    (collection, test) -> <n> Number of elements
+    collection <a|o|s> Collection
+    test <f> Test function
+        value <*> Value
+        key <n|s> Key
+        collection <a|o|s> Collection
+    > Ox.numberOf([0, 1, 0, 1], function(v) { return v; })
+    2
+    > Ox.numberOf({a: 'a', b: 'c'}, function(v, k) { return v == k; })
+    1
+    > Ox.numberOf('foo', function(v, k, c) { return v == c[k - 1]; })
+    1
+@*/
+Ox.numberOf = function(collection, test) {
+    return Ox.len(Ox.filter(collection, test));
+};
+
 /*@
 Ox.remove <f> Removes an element from an array or object and returns it
     (collection, element) -> <*> Element, or undefined if not found