diff --git a/SUMMARY.md b/SUMMARY.md
index 0a74aa0..d3af7fe 100644
--- a/SUMMARY.md
+++ b/SUMMARY.md
@@ -2,8 +2,6 @@
 
 * [Introduction](README.md)
 * [Core Principles](core-principles/README.md)
-    * [API First](core-principles/api-first.md)
-
-
-
+  * [API First](core-principles/api-first.md)
+  * [OpenAPI Specification](core-principles/openapi-specification.md)
 
diff --git a/core-principles/openapi-specification.md b/core-principles/openapi-specification.md
new file mode 100644
index 0000000..1bd2353
--- /dev/null
+++ b/core-principles/openapi-specification.md
@@ -0,0 +1,2 @@
+# OpenAPI Specification
+Every API has to be described using the [OpenAPI Specification (formerly known as Swagger Specification) version 2.0](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md).