diff --git a/.gitignore b/.gitignore
index 934db85..8251d56 100644
--- a/.gitignore
+++ b/.gitignore
@@ -90,3 +90,7 @@ typings/
 .dynamodb/
 
 dist/
+
+# Intellij IDEA Configuration
+.idea
+*.iml
diff --git a/src/transform-federation.ts b/src/transform-federation.ts
index c0f7d95..7e63f56 100644
--- a/src/transform-federation.ts
+++ b/src/transform-federation.ts
@@ -58,6 +58,10 @@ export function transformSchemaFederation<TContext>(
   const entityTypes = Object.fromEntries(
     Object.entries(federationConfig)
       .filter(([, { keyFields }]) => keyFields && keyFields.length)
+      .filter(([objectName]) => {
+          const type = schemaWithQueryType.getType(objectName);
+          return !isInterfaceType(type);
+      })
       .map(([objectName]) => {
         const type = schemaWithQueryType.getType(objectName);
         if (!isObjectType(type)) {
diff --git a/src/transform-sdl.ts b/src/transform-sdl.ts
index 21f595e..8ee1125 100644
--- a/src/transform-sdl.ts
+++ b/src/transform-sdl.ts
@@ -1,5 +1,5 @@
 import {
-  FieldDefinitionNode,
+  FieldDefinitionNode, InterfaceTypeDefinitionNode,
   ObjectTypeDefinitionNode,
   ObjectTypeExtensionNode,
   parse,
@@ -13,6 +13,7 @@ import {
   FederationFieldsConfig,
   FederationObjectConfig,
 } from './transform-federation';
+import { InterfaceTypeExtensionNode } from 'graphql';
 
 function createDirectiveWithFields(directiveName: string, fields: string) {
   return createDirectiveNode(directiveName, {
@@ -86,10 +87,45 @@ export function addFederationAnnotations<TContext>(
               )
             : [];
 
+          if (extend) {
+            newDirectives.push(createDirectiveNode('extends'))
+          }
+
+          return {
+            ...node,
+            directives: [...(node.directives || []), ...newDirectives],
+            kind: (extend ? 'ObjectTypeExtension' : node.kind)
+          };
+        }
+      },
+      leave() {
+        currentObjectName = undefined;
+      },
+    },
+    InterfaceTypeDefinition: {
+      enter(
+          node: InterfaceTypeDefinitionNode,
+      ): InterfaceTypeDefinitionNode | InterfaceTypeExtensionNode | undefined {
+        currentObjectName = node.name.value;
+        if (objectTypesTodo.has(currentObjectName)) {
+          objectTypesTodo.delete(currentObjectName);
+
+          const { keyFields, extend } = federationConfig[currentObjectName];
+
+          const newDirectives = keyFields
+              ? keyFields.map((keyField) =>
+                  createDirectiveWithFields('key', keyField),
+              )
+              : [];
+
+          if (extend) {
+            newDirectives.push(createDirectiveNode('extends'))
+          }
+
           return {
             ...node,
             directives: [...(node.directives || []), ...newDirectives],
-            kind: extend ? 'ObjectTypeExtension' : node.kind,
+            kind: (extend ? 'InterfaceTypeExtension' : node.kind)
           };
         }
       },