diff --git a/src/controllers/molecules/MessageComposer.js b/src/controllers/molecules/MessageComposer.js
index 11628bbf6..cdfee2eb8 100644
--- a/src/controllers/molecules/MessageComposer.js
+++ b/src/controllers/molecules/MessageComposer.js
@@ -19,10 +19,20 @@ limitations under the License.
 var MatrixClientPeg = require("../../MatrixClientPeg");
 
 var dis = require("../../dispatcher");
+var KeyCode = {
+    ENTER: 13,
+    TAB: 9,
+    SHIFT: 16
+};
 
 module.exports = {
-    componentDidMount: function() {
+    componentWillMount: function() {
         this.dispatcherRef = dis.register(this.onAction);
+        this.tabStruct = {
+            completing: false,
+            original: null,
+            index: 0
+        };
     },
 
     componentWillUnmount: function() {
@@ -38,30 +48,143 @@ module.exports = {
     },
 
     onKeyDown: function (ev) {
-        if (ev.keyCode == 13) {
-            var contentText = this.refs.textarea.getDOMNode().value;
-
-            var content = null;
-            if (/^\/me /i.test(contentText)) {
-                content = {
-                    msgtype: 'm.emote',
-                    body: contentText.substring(4)
-                };
-            } else {
-                content = {
-                    msgtype: 'm.text',
-                    body: contentText
-                };
+        if (ev.keyCode === KeyCode.ENTER) {
+            this.onEnter(ev);
+        }
+        else if (ev.keyCode === KeyCode.TAB) {
+            var members = [];
+            if (this.props.room) {
+                members = this.props.room.getJoinedMembers();
             }
-
-            MatrixClientPeg.get().sendMessage(this.props.room.roomId, content).then(function() {
-                dis.dispatch({
-                    action: 'message_sent'
-                });
-            });
-            this.refs.textarea.getDOMNode().value = '';
-            ev.preventDefault();
+            this.onTab(ev, members);
+        }
+        else if (ev.keyCode !== KeyCode.SHIFT && this.tabStruct.completing) {
+            // they're resuming typing; reset tab complete state vars.
+            this.tabStruct.completing = false;
+            this.tabStruct.index = 0;
         }
     },
+
+    onEnter: function(ev) {
+        var contentText = this.refs.textarea.getDOMNode().value;
+        var content = null;
+        if (/^\/me /i.test(contentText)) {
+            content = {
+                msgtype: 'm.emote',
+                body: contentText.substring(4)
+            };
+        } else {
+            content = {
+                msgtype: 'm.text',
+                body: contentText
+            };
+        }
+
+        MatrixClientPeg.get().sendMessage(this.props.room.roomId, content).then(function() {
+            dis.dispatch({
+                action: 'message_sent'
+            });
+        });
+        this.refs.textarea.getDOMNode().value = '';
+        ev.preventDefault();
+    },
+
+    onTab: function(ev, sortedMembers) {
+        var textArea = this.refs.textarea.getDOMNode();
+        if (!this.tabStruct.completing) {
+            this.tabStruct.completing = true;
+            this.tabStruct.index = 0;
+            // cache starting text
+            this.tabStruct.original = textArea.value;
+        }
+
+        // loop in the right direction
+        if (ev.shiftKey) {
+            this.tabStruct.index --;
+            if (this.tabStruct.index < 0) {
+                // wrap to the last search match, and fix up to a real index
+                // value after we've matched.
+                this.tabStruct.index = Number.MAX_VALUE;
+            }
+        }
+        else {
+            this.tabStruct.index++;
+        }
+
+        var searchIndex = 0;
+        var targetIndex = this.tabStruct.index;
+        var text = this.tabStruct.original;
+
+        var search = /@?([a-zA-Z0-9_\-:\.]+)$/.exec(text);
+        // console.log("Searched in '%s' - got %s", text, search);
+        if (targetIndex === 0) { // 0 is always the original text
+            textArea.value = text;
+        }
+        else if (search && search[1]) {
+            // console.log("search found: " + search+" from "+text);
+            var expansion;
+
+            // FIXME: could do better than linear search here
+            for (var i=0; i<sortedMembers.length; i++) {
+                var member = sortedMembers[i];
+                if (member.name && searchIndex < targetIndex) {
+                    if (member.name.toLowerCase().indexOf(search[1].toLowerCase()) === 0) {
+                        expansion = member.name;
+                        searchIndex++;
+                    }
+                }
+            }
+
+            if (searchIndex < targetIndex) { // then search raw mxids
+                for (var i=0; i<sortedMembers.length; i++) {
+                    if (searchIndex >= targetIndex) {
+                        break;
+                    }
+                    var userId = sortedMembers[i].userId;
+                    // === 1 because mxids are @username
+                    if (userId.toLowerCase().indexOf(search[1].toLowerCase()) === 1) {
+                        expansion = userId;
+                        searchIndex++;
+                    }
+                }
+            }
+
+            if (searchIndex === targetIndex ||
+                    targetIndex === Number.MAX_VALUE) {
+                // xchat-style tab complete, add a colon if tab
+                // completing at the start of the text
+                if (search[0].length === text.length) {
+                    expansion += ": ";
+                }
+                else {
+                    expansion += " ";
+                }
+                textArea.value = text.replace(
+                    /@?([a-zA-Z0-9_\-:\.]+)$/, expansion
+                );
+                // cancel blink
+                textArea.style["background-color"] = "";
+                if (targetIndex === Number.MAX_VALUE) {
+                    // wrap the index around to the last index found
+                    this.tabStruct.index = searchIndex;
+                    targetIndex = searchIndex;
+                }
+            }
+            else {
+                // console.log("wrapped!");
+                textArea.style["background-color"] = "#faa";
+                setTimeout(function() {
+                     textArea.style["background-color"] = "";
+                }, 150);
+                textArea.value = text;
+                this.tabStruct.index = 0;
+            }
+        }
+        else {
+            this.tabStruct.index = 0;
+        }
+        // prevent the default TAB operation (typically focus shifting)
+        ev.preventDefault();
+    }
 };