diff --git a/test/arrays.js b/test/arrays.js
index 80da71f6f..c648f081b 100644
--- a/test/arrays.js
+++ b/test/arrays.js
@@ -50,10 +50,13 @@ $(document).ready(function() {
   });
 
   test("arrays: flatten", function() {
-    var list = [1, [2], [3, [[[4]]]]];
-    equals(_.flatten(list).join(', '), '1, 2, 3, 4', 'can flatten nested arrays');
-    var result = (function(){ return _.flatten(arguments); })(1, [2], [3, [[[4]]]]);
-    equals(result.join(', '), '1, 2, 3, 4', 'works on an arguments object');
+    if (window.JSON) {
+      var list = [1, [2], [3, [[[4]]]]];
+      equals(JSON.stringify(_.flatten(list)), '[1,2,3,4]', 'can flatten nested arrays');
+      equals(JSON.stringify(_.flatten(list, true)), '[1,2,3,[[[4]]]]', 'can shallowly flatten nested arrays');
+      var result = (function(){ return _.flatten(arguments); })(1, [2], [3, [[[4]]]]);
+      equals(JSON.stringify(result), '[1,2,3,4]', 'works on an arguments object');
+    }
   });
 
   test("arrays: without", function() {
@@ -97,6 +100,9 @@ $(document).ready(function() {
   test("arrays: union", function() {
     var result = _.union([1, 2, 3], [2, 30, 1], [1, 40]);
     equals(result.join(' '), '1 2 3 30 40', 'takes the union of a list of arrays');
+
+    var result = _.union([1, 2, 3], [2, 30, 1], [1, 40, [1]]);
+    equals(result.join(' '), '1 2 3 30 40 1', 'takes the union of a list of nested arrays');
   });
 
   test("arrays: difference", function() {
diff --git a/underscore.js b/underscore.js
index f2c8c94b7..f2f42898f 100644
--- a/underscore.js
+++ b/underscore.js
@@ -345,9 +345,9 @@
   };
 
   // Return a completely flattened version of an array.
-  _.flatten = function(array) {
+  _.flatten = function(array, shallow) {
     return _.reduce(array, function(memo, value) {
-      if (_.isArray(value)) return memo.concat(_.flatten(value));
+      if (_.isArray(value)) return memo.concat(shallow ? value : _.flatten(value));
       memo[memo.length] = value;
       return memo;
     }, []);
@@ -377,7 +377,7 @@
   // Produce an array that contains the union: each distinct element from all of
   // the passed-in arrays.
   _.union = function() {
-    return _.uniq(_.flatten(arguments));
+    return _.uniq(_.flatten(arguments, true));
   };
 
   // Produce an array that contains every item shared between all the