diff --git a/include/gsl/gsl b/include/gsl/gsl
index 55862eb..6415d48 100644
--- a/include/gsl/gsl
+++ b/include/gsl/gsl
@@ -21,6 +21,7 @@
 #include <gsl/gsl_assert>    // Ensures/Expects
 #include <gsl/gsl_byte>      // byte
 #include <gsl/gsl_util>      // finally()/narrow()/narrow_cast()...
+#include <gsl/index>         // index
 #include <gsl/multi_span>    // multi_span, strided_span...
 #include <gsl/pointers>      // owner, not_null
 #include <gsl/span>          // span
diff --git a/include/gsl/index b/include/gsl/index
new file mode 100644
index 0000000..176860c
--- /dev/null
+++ b/include/gsl/index
@@ -0,0 +1,27 @@
+///////////////////////////////////////////////////////////////////////////////
+//
+// Copyright (c) 2015 Microsoft Corporation. All rights reserved.
+//
+// This code is licensed under the MIT License (MIT).
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef GSL_INDEX_H
+#define GSL_INDEX_H
+
+#include <cstddef> // for ptrdiff_t
+
+namespace gsl
+{
+using index = std::ptrdiff_t;
+}
+
+#endif // GSL_INDEX_H
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 86ce5a4..544d1cc 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -101,3 +101,4 @@ add_gsl_test(utils_tests)
 add_gsl_test(owner_tests)
 add_gsl_test(byte_tests)
 add_gsl_test(algorithm_tests)
+add_gsl_test(index_test)
diff --git a/tests/index_test.cpp b/tests/index_test.cpp
new file mode 100644
index 0000000..761e70f
--- /dev/null
+++ b/tests/index_test.cpp
@@ -0,0 +1,26 @@
+///////////////////////////////////////////////////////////////////////////////
+//
+// Copyright (c) 2015 Microsoft Corporation. All rights reserved.
+//
+// This code is licensed under the MIT License (MIT).
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+///////////////////////////////////////////////////////////////////////////////
+
+#include <catch/catch.hpp>
+#include <type_traits> // for is_same
+
+#include <gsl/index> // for index
+
+TEST_CASE("sanity check for gsl::index typedef")
+{
+    static_assert(std::is_same<gsl::index, std::ptrdiff_t>::value,
+                  "gsl::index represents wrong arithmetic type");
+}