diff --git a/sky/packages/sky_services/BUILD.gn b/sky/packages/sky_services/BUILD.gn
index add15dc074f..69007ec819f 100644
--- a/sky/packages/sky_services/BUILD.gn
+++ b/sky/packages/sky_services/BUILD.gn
@@ -29,6 +29,7 @@ dart_pkg("sky_services") {
     "//sky/services/media:interfaces",
     "//sky/services/pointer:interfaces",
     "//sky/services/raw_keyboard:interfaces",
+    "//sky/services/semantics:interfaces",
     "//sky/services/updater:interfaces",
   ]
 }
diff --git a/sky/services/semantics/BUILD.gn b/sky/services/semantics/BUILD.gn
new file mode 100644
index 00000000000..41d83dd7495
--- /dev/null
+++ b/sky/services/semantics/BUILD.gn
@@ -0,0 +1,19 @@
+# Copyright 2015 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//mojo/public/tools/bindings/mojom.gni")
+
+group("semantics") {
+  testonly = true
+
+  deps = [
+    ":interfaces",
+  ]
+}
+
+mojom("interfaces") {
+  sources = [
+    "semantics.mojom",
+  ]
+}
diff --git a/sky/services/semantics/semantics.mojom b/sky/services/semantics/semantics.mojom
new file mode 100644
index 00000000000..6ef1a89b1a9
--- /dev/null
+++ b/sky/services/semantics/semantics.mojom
@@ -0,0 +1,69 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+[DartPackage="sky_services"]
+module semantics;
+
+struct SemanticsNode {
+  uint32 id;
+  SemanticFlags? flags;
+  SemanticStrings? strings;
+  SemanticGeometry? geometry;
+  array<SemanticsNode> children;
+};
+
+struct SemanticFlags {
+  // This is intended to just be booleans, so that it can be extended
+  // over time yet still be packed tightly.
+  bool canBeTapped = false;
+  bool canBeLongPressed = false;
+  bool canBeScrolledHorizontally = false;
+  bool canBeScrolledVertically = false;
+  bool hasCheckedState = false; // whether isChecked is relevant
+  bool isChecked = false;
+};
+
+struct SemanticStrings {
+  // This is intended to just be Strings.
+  string label;
+};
+
+struct SemanticGeometry {
+  // A SemanticsNode defines a rectangle in a cartesian coordinate
+  // space within which it is potentially interactive.
+
+  // The transform from the coordinate space of the parent
+  // SemanticsNode (or the application's display area, if this is the
+  // root node) to this SemanticsNode's coordinate space, described as
+  // a 4x4 matrix, with values in column-major order. The array must
+  // have exactly 16 values.
+  array<float> transform;
+
+  // The position and size of the potentially interactive rectangle in the
+  // SemanticsNode's coordinate space.
+  // If the width or height are zero, then the node should be treated
+  // as absent for most purposes. The node may, however, have further
+  // children.
+  float left;
+  float top;
+  float width;
+  float height;
+};
+
+[ServiceName="semantics::SemanticsClient"]
+interface SemanticsClient {
+  // The OS side, invoked from the app.
+  UpdateSemanticsTree(array<SemanticsNode> nodes);
+};
+
+[ServiceName="semantics::SemanticsServer"]
+interface SemanticsServer {
+  // The app side, invoked from the engine.
+  Tap(uint32 nodeID);
+  LongPress(uint32 nodeID);
+  ScrollLeft(uint32 nodeID);
+  ScrollRight(uint32 nodeID);
+  ScrollUp(uint32 nodeID);
+  ScrollDown(uint32 nodeID);
+};