diff --git a/src/cljs/clojure/data.cljs b/src/cljs/clojure/data.cljs
new file mode 100644
index 0000000000..11a84af313
--- /dev/null
+++ b/src/cljs/clojure/data.cljs
@@ -0,0 +1,162 @@
+;   Copyright (c) Rich Hickey. All rights reserved.
+;   The use and distribution terms for this software are covered by the
+;   Eclipse Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php)
+;   which can be found in the file epl-v10.html at the root of this distribution.
+;   By using this software in any fashion, you are agreeing to be bound by
+;   the terms of this license.
+;   You must not remove this notice, or any other, from this software.
+
+(ns 
+  ^{:author "Stuart Halloway",
+    :doc "Non-core data functions."}
+  clojure.data
+  (:require [clojure.set :as set]))
+
+(declare diff)
+
+(defn- atom-diff
+  "Internal helper for diff."
+  [a b]
+  (if (= a b) [nil nil a] [a b nil]))
+
+;; for big things a sparse vector class would be better
+(defn- vectorize
+  "Convert an associative-by-numeric-index collection into
+   an equivalent vector, with nil for any missing keys"
+  [m]
+  (when (seq m)
+    (reduce
+     (fn [result [k v]] (assoc result k v))
+     (vec (repeat (apply max (keys m))  nil))
+     m)))
+
+(defn- diff-associative-key
+  "Diff associative things a and b, comparing only the key k."
+  [a b k]
+  (let [va (get a k)
+        vb (get b k)
+        [a* b* ab] (diff va vb)
+        in-a (contains? a k)
+        in-b (contains? b k)
+        same (and in-a in-b
+                  (or (not (nil? ab))
+                      (and (nil? va) (nil? vb))))]
+    [(when (and in-a (or (not (nil? a*)) (not same))) {k a*})
+     (when (and in-b (or (not (nil? b*)) (not same))) {k b*})
+     (when same {k ab})
+     ]))
+
+(defn- diff-associative
+  "Diff associative things a and b, comparing only keys in ks (if supplied)."
+  ([a b]
+     (diff-associative a b (set/union (keys a) (keys b))))
+  ([a b ks]
+     (reduce
+      (fn [diff1 diff2]
+        (doall (map merge diff1 diff2)))
+      [nil nil nil]
+      (map
+       (partial diff-associative-key a b)
+       ks))))
+
+(defn- diff-sequential
+  [a b]
+  (vec (map vectorize (diff-associative
+                       (if (vector? a) a (vec a))
+                       (if (vector? b) b (vec b))
+                       (range (max (count a) (count b)))))))
+
+(defn- diff-set
+  [a b]
+  [(not-empty (set/difference a b))
+   (not-empty (set/difference b a))
+   (not-empty (set/intersection a b))])
+
+(defprotocol EqualityPartition
+  "Implementation detail. Subject to change."
+  (equality-partition [x] "Implementation detail. Subject to change."))
+
+(defprotocol Diff
+  "Implementation detail. Subject to change."
+  (diff-similar [a b] "Implementation detail. Subject to change."))
+
+(extend-protocol EqualityPartition
+  nil
+  (equality-partition [x] :atom)
+
+  string
+  (equality-partition [x] :atom)
+
+  number
+  (equality-partition [x] :atom)
+
+  array
+  (equality-partition [x] :sequential)
+
+  function
+  (equality-partition [x] :atom)
+
+  boolean
+  (equality-partition [x] :atom)
+
+  default
+  (equality-partition [x]
+    (cond
+     (satisfies? IMap x) :map
+     (satisfies? ISet x) :set
+     (satisfies? ISequential x) :sequential
+     :default :atom)))
+
+(extend-protocol Diff
+  nil
+  (diff-similar [a b]
+    (atom-diff a b))
+
+  string
+  (diff-similar [a b]
+    (atom-diff a b))
+
+  number
+  (diff-similar [a b]
+    (atom-diff a b))
+
+  array
+  (diff-similar [a b]
+    (diff-sequential a b))
+
+  function
+  (diff-similar [a b]
+    (atom-diff a b))
+
+  boolean
+  (diff-similar [a b]
+    (atom-diff a b))
+
+  default
+  (diff-similar [a b]
+    ((case (equality-partition a)
+       :atom atom-diff
+       :set diff-set
+       :sequential diff-sequential
+       :map diff-associative)
+     a b)))
+
+(defn diff
+  "Recursively compares a and b, returning a tuple of
+  [things-only-in-a things-only-in-b things-in-both].
+  Comparison rules:
+
+  * For equal a and b, return [nil nil a].
+  * Maps are subdiffed where keys match and values differ.
+  * Sets are never subdiffed.
+  * All sequential things are treated as associative collections
+    by their indexes, with results returned as vectors.
+  * Everything else (including strings!) is treated as
+    an atom and compared for equality."
+  [a b]
+  (if (= a b)
+    [nil nil a]
+    (if (= (equality-partition a) (equality-partition b))
+      (diff-similar a b)
+      (atom-diff a b))))
+  
diff --git a/test/cljs/clojure/data_test.cljs b/test/cljs/clojure/data_test.cljs
new file mode 100644
index 0000000000..009b1ccbd5
--- /dev/null
+++ b/test/cljs/clojure/data_test.cljs
@@ -0,0 +1,22 @@
+(ns clojure.data-test
+  (:require [clojure.data :refer [diff]]))
+
+(defn test-data []
+  (assert (= [nil nil nil] (diff nil nil)))
+  (assert (= [1 2 nil] (diff 1 2)))
+  (assert (= [nil nil [1 2 3]] (diff [1 2 3] '(1 2 3))))
+  (assert (= [1 [:a :b] nil] (diff 1 [:a :b])))
+  (assert (= [{:a 1} :b nil] (diff {:a 1} :b)))
+  (assert (= [:team #{:p1 :p2} nil] (diff :team #{:p1 :p2})))
+  (assert (= [{0 :a} [:a] nil] (diff {0 :a} [:a])))
+  (assert (= [nil [nil 2] [1]] (diff [1] [1 2])))
+  (assert (= [nil nil [1 2]] (diff [1 2] (into-array [1 2]))))
+  (assert (= [#{:a} #{:b} #{:c :d}] (diff #{:a :c :d} #{:b :c :d})))
+  (assert (= [nil nil {:a 1}] (diff {:a 1} {:a 1})))
+  (assert (= [{:a #{2}} {:a #{4}} {:a #{3}}] (diff {:a #{2 3}} {:a #{3 4}})))
+  (assert (= [nil nil [1 2]] (diff [1 2] (into-array [1 2]))))
+  (assert (= [nil nil [1 2]] (diff (into-array [1 2]) [1 2])))
+  (assert (= [{:a {:c [1]}} {:a {:c [0]}} {:a {:c [nil 2] :b 1}}]
+             (diff {:a {:b 1 :c [1 2]}} {:a {:b 1 :c [0 2]}})))
+  (assert (= [{:a nil} {:a false} {:b nil :c false}]
+             (diff {:a nil :b nil :c false} {:a false :b nil :c false}))))
diff --git a/test/cljs/test_runner.cljs b/test/cljs/test_runner.cljs
index fa6b14f7ca..7b2c23be52 100644
--- a/test/cljs/test_runner.cljs
+++ b/test/cljs/test_runner.cljs
@@ -4,6 +4,7 @@
             [cljs.binding-test :as binding-test]
             [cljs.ns-test :as ns-test]
             [clojure.string-test :as string-test]
+            [clojure.data-test :as data-test]
             [cljs.macro-test :as macro-test]
             [cljs.letfn-test :as letfn-test]
             [foo.ns-shadow-test :as ns-shadow-test]))
@@ -13,6 +14,7 @@
 (core-test/test-stuff)
 (reader-test/test-reader)
 (string-test/test-string)
+(data-test/test-data)
 (binding-test/test-binding)
 (ns-test/test-ns)
 (macro-test/test-macros)