diff --git a/examples/unfinished/shark/client/shark.js b/examples/unfinished/shark/client/shark.js
index 8aa1117ded..3de8da2051 100644
--- a/examples/unfinished/shark/client/shark.js
+++ b/examples/unfinished/shark/client/shark.js
@@ -46,4 +46,43 @@ Template.item({
 //  $(table).append(frag);
 //
 //  console.log(TABLE.rows.length);
-//});
\ No newline at end of file
+//});
+
+Span = UIComponent.extend({
+  render: function (buf) {
+    buf("<span style='background:red;margin:5px'>Hello</span>");
+  }
+});
+
+Div = UIComponent.extend({
+  render: function (buf) {
+    buf("<div style='background:blue;margin:5px'>World</div>");
+  }
+});
+
+Either = UIComponent.extend({
+  render: function (buf) {
+    buf(Div.create(),
+        {
+          type: function () { return window[Session.get('which')]; },
+          args: {
+            built: function () {
+              var self = this;
+              self.$("*").on('click', function (evt) {
+                Session.set(
+                  'which',
+                  Session.get('which') === 'Div' ? 'Span' : 'Div');
+              });
+            }
+          }
+        },
+        { type: Span });
+  }
+});
+
+Meteor.startup(function () {
+  Session.set('which', 'Span');
+
+  var x = Either.create({isRoot: true});
+  x.attach(document.body);
+});
diff --git a/packages/ui/dom.js b/packages/ui/dom.js
index 74f62e9d23..cf304c5d70 100644
--- a/packages/ui/dom.js
+++ b/packages/ui/dom.js
@@ -94,33 +94,16 @@ Component({
   _populate: function (div) {
     var self = this;
 
-    var strs = [];
-    var randomString = Random.id();
-    var commentUid = 1;
-    var componentsToAttach = {};
+    var buf = makeRenderBuffer(self);
+    self.render(buf);
 
-    self.render(function (/*args*/) {
-      for (var i = 0; i < arguments.length; i++) {
-        var arg = arguments[i];
-        if (typeof arg === 'string') {
-          strs.push(arg);
-        } else if (arg instanceof Component) {
-          var commentString = randomString + '_' + (commentUid++);
-          strs.push('<!--', commentString, '-->');
-          self.add(arg);
-          componentsToAttach[commentString] = arg;
-        } else {
-          throw new Error("Expected string or Component");
-        }
-      }
-    });
-
-    var html = strs.join('');
+    var html = buf.getHtml();
 
     $(div).append(html);
     var start = div.firstChild;
     var end = div.lastChild;
 
+    var componentsToAttach = buf.componentsToAttach;
     // walk div and replace comments with Components
 
     var wireUpDOM = function (parent) {
@@ -182,6 +165,7 @@ Component({
 
       if (c.firstRun) {
         var div = makeSafeDiv();
+        // capture reactivity:
         var info = self._populate(div);
 
         if (! div.firstChild)
@@ -191,6 +175,7 @@ Component({
         self.start = info.start || div.firstChild;
         self.end = info.end || div.lastChild;
       } else {
+        // capture reactivity:
         self._rebuild(c.builtChildren);
       }
 
@@ -593,6 +578,38 @@ Component({
     self._computations.push(c);
 
     return c;
+  },
+
+  replaceChild: function (oldChild, newChild) {
+    var self = this;
+
+    self._requireBuilt();
+    oldChild._requireBuilt();
+    if (! oldChild.isAttached)
+      throw new Error("Child to replace must be attached");
+
+    var lastNode = oldChild.lastNode();
+    var parentNode = lastNode.parentNode;
+    var nextNode = lastNode.nextSibling;
+
+    oldChild.remove();
+    self.insertBefore(newChild, nextNode, parentNode);
+  },
+
+  swapChild: function (oldChild, newChild) {
+    var self = this;
+
+    self._requireBuilt();
+    oldChild._requireBuilt();
+    if (! oldChild.isAttached)
+      throw new Error("Child to swap out must be attached");
+
+    var lastNode = oldChild.lastNode();
+    var parentNode = lastNode.parentNode;
+    var nextNode = lastNode.nextSibling;
+
+    oldChild.detach();
+    self.insertBefore(newChild, nextNode, parentNode);
   }
 
   // If Component is ever emptied, it gets an empty comment node.
diff --git a/packages/ui/package.js b/packages/ui/package.js
index 350cc55de8..abf7df835a 100644
--- a/packages/ui/package.js
+++ b/packages/ui/package.js
@@ -16,6 +16,7 @@ Package.on_use(function (api) {
   api.add_files(['base.js',
                  'lifecycle.js',
                  'tree.js',
+                 'render.js',
                  'dom.js',
                  'forms.js',
                  'components.js',
diff --git a/packages/ui/render.js b/packages/ui/render.js
new file mode 100644
index 0000000000..22eebe2a0c
--- /dev/null
+++ b/packages/ui/render.js
@@ -0,0 +1,65 @@
+var Component = UIComponent;
+
+
+makeRenderBuffer = function (component, options) {
+  var isPreview = !! options && options.preview;
+
+  var strs = [];
+  var componentsToAttach = {};
+  var randomString = Random.id();
+  var commentUid = 1;
+
+  var handle = function (arg) {
+    if (typeof arg === 'string') {
+      strs.push(arg);
+    } else if (arg instanceof Component) {
+      var commentString = randomString + '_' + (commentUid++);
+      strs.push('<!--', commentString, '-->');
+      component.add(arg);
+      componentsToAttach[commentString] = arg;
+    } else if (arg.type) {
+      // `{type: componentTypeOrFunction, args: object}`
+      if (Component.isType(arg.type)) {
+        handle(arg.type.create(arg.args));
+      } else if (typeof arg.type === 'function') {
+        var curType;
+        component.autorun(function (c) {
+          // capture reactivity:
+          var type = arg.type();
+          if (c.firstRun) {
+            curType = type;
+          } else if (component.stage !== Component.BUILT ||
+                     ! component.hasChild(curChild)) {
+            c.stop();
+          } else if (type !== curType) {
+            var oldChild = curChild;
+            curType = type;
+            Deps.nonreactive(function () {
+              curChild = curType.create(arg.args);
+              component.replaceChild(oldChild, curChild);
+            });
+          }
+        });
+        var curChild = curType.create(arg.args);
+        handle(curChild);
+      } else {
+        throw new Error("Expected 'type' to be Component or function");
+      }
+    } else {
+      throw new Error("Expected string or Component");
+    }
+  };
+
+  var buf = function (/*args*/) {
+    for (var i = 0; i < arguments.length; i++)
+      handle(arguments[i]);
+  };
+
+  buf.getHtml = function () {
+    return strs.join('');
+  };
+
+  buf.componentsToAttach = componentsToAttach;
+
+  return buf;
+};
diff --git a/packages/ui/tree.js b/packages/ui/tree.js
index 76afa555b6..804a6328be 100644
--- a/packages/ui/tree.js
+++ b/packages/ui/tree.js
@@ -132,6 +132,12 @@ Component({
     self._added();
   },
 
+  hasChild: function (comp) {
+    this._requireNotDestroyed();
+
+    return this.children[comp.guid] === comp;
+  },
+
   extendHooks: {
     isRoot: function (value) {
       if (value)