diff --git a/js/id/actions/merge_remote_changes.js b/js/id/actions/merge_remote_changes.js
index bfcb78839..5eb7eba91 100644
--- a/js/id/actions/merge_remote_changes.js
+++ b/js/id/actions/merge_remote_changes.js
@@ -1,14 +1,74 @@
-/* jshint ignore:start */
 iD.actions.MergeRemoteChanges = function(base, local, remote) {
+    var option = 'safe',  // 'safe', 'force_local', 'force_remote'
+        target;
+
+    function assertIds() {
+        return (base.id === local.id) && (base.id === remote.id);
+    }
+
+    function sameLocation() {
+        var epsilon = 1e-6;
+        return (Math.abs(remote.loc[0] - local.loc[0]) < epsilon) &&
+                (Math.abs(remote.loc[1] - local.loc[1]) < epsilon);
+    }
+
+    function mergeChildren() {
+        // todo, support non-destructive merging
+        return _.isEqual(local.nodes, remote.nodes);
+    }
+
+    function mergeMembers() {
+        // todo, support non-destructive merging
+        return _.isEqual(local.members, remote.members);
+    }
+
+    function mergeTags() {
+        var keys = _.reject(_.union(_.keys(base.tags), _.keys(remote.tags)), ignoreKey),
+            tags = _.cloneDeep(target.tags);
+
+        function ignoreKey(k) {
+            return k.indexOf('tiger:') === 0 || _.contains(iD.data.discarded, k);
+        }
+
+        for (var i = 0, imax = keys.length; i !== imax; i++) {
+            var k = keys[i];
+            if (remote.tags[k] !== base.tags[k]) {  // tag modified remotely..
+                if (local.tags[k] && local.tags[k] !== remote.tags[k]) {
+                    return false;
+                } else {
+                    tags[k] = remote.tags[k];
+                }
+            }
+        }
+
+        target = target.update({tags: tags});
+        return true;
+    }
 
     var action = function(graph) {
+        if (!assertIds()) { return graph; }
 
-        // TODO
-        debugger;
+        target = iD.Entity(local, {version: remote.version});
+        if (option === 'force_remote') { return graph.replace(remote); }
+        if (option === 'force_local') { return graph.replace(target); }
 
-        return graph;
+        // otherwise, safe mode: only permit non-destructive merges..
+        var doMerge;
+        if (target.type === 'node') {
+            doMerge = (sameLocation() && mergeTags());
+        } else if (target.type === 'way') {
+            doMerge = (mergeChildren() && mergeTags());
+        } else if (target.type === 'relation') {
+            doMerge = (mergeMembers() && mergeTags());
+        }
+
+        return doMerge ? graph.replace(target) : graph;
+    };
+
+    action.withOption = function(opt) {
+        option = opt;
+        return action;
     };
 
     return action;
 };
-/* jshint ignore:end */