diff --git a/src/Language/AST/EnumTypeDefinitionNode.php b/src/Language/AST/EnumTypeDefinitionNode.php
index 71ca508..e9be727 100644
--- a/src/Language/AST/EnumTypeDefinitionNode.php
+++ b/src/Language/AST/EnumTypeDefinitionNode.php
@@ -19,7 +19,7 @@ class EnumTypeDefinitionNode extends Node implements TypeDefinitionNode
     public $directives;
 
     /**
-     * @var EnumValueDefinitionNode[]
+     * @var EnumValueDefinitionNode[]|null
      */
     public $values;
 
diff --git a/src/Language/AST/EnumTypeExtensionNode.php b/src/Language/AST/EnumTypeExtensionNode.php
new file mode 100644
index 0000000..5e2417d
--- /dev/null
+++ b/src/Language/AST/EnumTypeExtensionNode.php
@@ -0,0 +1,25 @@
+<?php
+namespace GraphQL\Language\AST;
+
+class EnumTypeExtensionNode extends Node implements TypeExtensionNode
+{
+    /**
+     * @var string
+     */
+    public $kind = NodeKind::ENUM_TYPE_EXTENSION;
+
+    /**
+     * @var NameNode
+     */
+    public $name;
+
+    /**
+     * @var DirectiveNode[]|null
+     */
+    public $directives;
+
+    /**
+     * @var EnumValueDefinitionNode[]|null
+     */
+    public $values;
+}
diff --git a/src/Language/AST/InputObjectTypeDefinitionNode.php b/src/Language/AST/InputObjectTypeDefinitionNode.php
index c56ceca..b4838c7 100644
--- a/src/Language/AST/InputObjectTypeDefinitionNode.php
+++ b/src/Language/AST/InputObjectTypeDefinitionNode.php
@@ -14,12 +14,12 @@ class InputObjectTypeDefinitionNode extends Node implements TypeDefinitionNode
     public $name;
 
     /**
-     * @var DirectiveNode[]
+     * @var DirectiveNode[]|null
      */
     public $directives;
 
     /**
-     * @var InputValueDefinitionNode[]
+     * @var InputValueDefinitionNode[]|null
      */
     public $fields;
 
diff --git a/src/Language/AST/InputObjectTypeExtensionNode.php b/src/Language/AST/InputObjectTypeExtensionNode.php
new file mode 100644
index 0000000..d511d09
--- /dev/null
+++ b/src/Language/AST/InputObjectTypeExtensionNode.php
@@ -0,0 +1,25 @@
+<?php
+namespace GraphQL\Language\AST;
+
+class InputObjectTypeExtensionNode extends Node implements TypeExtensionNode
+{
+    /**
+     * @var string
+     */
+    public $kind = NodeKind::INPUT_OBJECT_TYPE_EXTENSION;
+
+    /**
+     * @var NameNode
+     */
+    public $name;
+
+    /**
+     * @var DirectiveNode[]|null
+     */
+    public $directives;
+
+    /**
+     * @var InputValueDefinitionNode[]|null
+     */
+    public $fields;
+}
diff --git a/src/Language/AST/InterfaceTypeDefinitionNode.php b/src/Language/AST/InterfaceTypeDefinitionNode.php
index ff9bc1f..44aebd0 100644
--- a/src/Language/AST/InterfaceTypeDefinitionNode.php
+++ b/src/Language/AST/InterfaceTypeDefinitionNode.php
@@ -14,14 +14,14 @@ class InterfaceTypeDefinitionNode extends Node implements TypeDefinitionNode
     public $name;
 
     /**
-     * @var DirectiveNode[]
+     * @var DirectiveNode[]|null
      */
     public $directives;
 
     /**
-     * @var FieldDefinitionNode[]
+     * @var FieldDefinitionNode[]|null
      */
-    public $fields = [];
+    public $fields;
 
     /**
      * @var StringValueNode|null
diff --git a/src/Language/AST/InterfaceTypeExtensionNode.php b/src/Language/AST/InterfaceTypeExtensionNode.php
new file mode 100644
index 0000000..2cb5b96
--- /dev/null
+++ b/src/Language/AST/InterfaceTypeExtensionNode.php
@@ -0,0 +1,25 @@
+<?php
+namespace GraphQL\Language\AST;
+
+class InterfaceTypeExtensionNode extends Node implements TypeExtensionNode
+{
+    /**
+     * @var string
+     */
+    public $kind = NodeKind::INTERFACE_TYPE_EXTENSION;
+
+    /**
+     * @var NameNode
+     */
+    public $name;
+
+    /**
+     * @var DirectiveNode[]|null
+     */
+    public $directives;
+
+    /**
+     * @var FieldDefinitionNode[]|null
+     */
+    public $fields;
+}
diff --git a/src/Language/AST/NodeKind.php b/src/Language/AST/NodeKind.php
index 22287c4..1091c6e 100644
--- a/src/Language/AST/NodeKind.php
+++ b/src/Language/AST/NodeKind.php
@@ -65,7 +65,12 @@ class NodeKind
 
     // Type Extensions
 
+    const SCALAR_TYPE_EXTENSION = 'ScalarTypeExtension';
     const OBJECT_TYPE_EXTENSION = 'ObjectTypeExtension';
+    const INTERFACE_TYPE_EXTENSION = 'InterfaceTypeExtension';
+    const UNION_TYPE_EXTENSION = 'UnionTypeExtension';
+    const ENUM_TYPE_EXTENSION = 'EnumTypeExtension';
+    const INPUT_OBJECT_TYPE_EXTENSION = 'InputObjectTypeExtension';
 
     // Directive Definitions
 
@@ -127,7 +132,12 @@ class NodeKind
         NodeKind::INPUT_OBJECT_TYPE_DEFINITION =>InputObjectTypeDefinitionNode::class,
 
         // Type Extensions
+        NodeKind::SCALAR_TYPE_EXTENSION => ScalarTypeExtensionNode::class,
         NodeKind::OBJECT_TYPE_EXTENSION => ObjectTypeExtensionNode::class,
+        NodeKind::INTERFACE_TYPE_EXTENSION => InterfaceTypeExtensionNode::class,
+        NodeKind::UNION_TYPE_EXTENSION => UnionTypeExtensionNode::class,
+        NodeKind::ENUM_TYPE_EXTENSION => EnumTypeExtensionNode::class,
+        NodeKind::INPUT_OBJECT_TYPE_EXTENSION => InputObjectTypeExtensionNode::class,
 
         // Directive Definitions
         NodeKind::DIRECTIVE_DEFINITION => DirectiveDefinitionNode::class
diff --git a/src/Language/AST/ObjectTypeDefinitionNode.php b/src/Language/AST/ObjectTypeDefinitionNode.php
index addf20a..b2c6b1b 100644
--- a/src/Language/AST/ObjectTypeDefinitionNode.php
+++ b/src/Language/AST/ObjectTypeDefinitionNode.php
@@ -19,12 +19,12 @@ class ObjectTypeDefinitionNode extends Node implements TypeDefinitionNode
     public $interfaces = [];
 
     /**
-     * @var DirectiveNode[]
+     * @var DirectiveNode[]|null
      */
     public $directives;
 
     /**
-     * @var FieldDefinitionNode[]
+     * @var FieldDefinitionNode[]|null
      */
     public $fields;
 
diff --git a/src/Language/AST/ScalarTypeExtensionNode.php b/src/Language/AST/ScalarTypeExtensionNode.php
new file mode 100644
index 0000000..1fc2d82
--- /dev/null
+++ b/src/Language/AST/ScalarTypeExtensionNode.php
@@ -0,0 +1,20 @@
+<?php
+namespace GraphQL\Language\AST;
+
+class ScalarTypeExtensionNode extends Node implements TypeExtensionNode
+{
+    /**
+     * @var string
+     */
+    public $kind = NodeKind::SCALAR_TYPE_EXTENSION;
+
+    /**
+     * @var NameNode
+     */
+    public $name;
+
+    /**
+     * @var DirectiveNode[]|null
+     */
+    public $directives;
+}
diff --git a/src/Language/AST/TypeExtensionNode.php b/src/Language/AST/TypeExtensionNode.php
index db50f2a..a37b9de 100644
--- a/src/Language/AST/TypeExtensionNode.php
+++ b/src/Language/AST/TypeExtensionNode.php
@@ -5,6 +5,11 @@ interface TypeExtensionNode extends TypeSystemDefinitionNode
 {
     /**
     export type TypeExtensionNode =
-      | ObjectTypeExtensionNode;
+      | ScalarTypeExtensionNode
+      | ObjectTypeExtensionNode
+      | InterfaceTypeExtensionNode
+      | UnionTypeExtensionNode
+      | EnumTypeExtensionNode
+      | InputObjectTypeExtensionNode;
      */
 }
diff --git a/src/Language/AST/UnionTypeDefinitionNode.php b/src/Language/AST/UnionTypeDefinitionNode.php
index 0eae11a..46c1151 100644
--- a/src/Language/AST/UnionTypeDefinitionNode.php
+++ b/src/Language/AST/UnionTypeDefinitionNode.php
@@ -19,9 +19,9 @@ class UnionTypeDefinitionNode extends Node implements TypeDefinitionNode
     public $directives;
 
     /**
-     * @var NamedTypeNode[]
+     * @var NamedTypeNode[]|null
      */
-    public $types = [];
+    public $types;
 
     /**
      * @var StringValueNode|null
diff --git a/src/Language/AST/UnionTypeExtensionNode.php b/src/Language/AST/UnionTypeExtensionNode.php
new file mode 100644
index 0000000..e605010
--- /dev/null
+++ b/src/Language/AST/UnionTypeExtensionNode.php
@@ -0,0 +1,25 @@
+<?php
+namespace GraphQL\Language\AST;
+
+class UnionTypeExtensionNode extends Node implements TypeExtensionNode
+{
+    /**
+     * @var string
+     */
+    public $kind = NodeKind::UNION_TYPE_EXTENSION;
+
+    /**
+     * @var NameNode
+     */
+    public $name;
+
+    /**
+     * @var DirectiveNode[]|null
+     */
+    public $directives;
+
+    /**
+     * @var NamedTypeNode[]|null
+     */
+    public $types;
+}
diff --git a/src/Language/Parser.php b/src/Language/Parser.php
index b5a16fc..b843f3d 100644
--- a/src/Language/Parser.php
+++ b/src/Language/Parser.php
@@ -4,11 +4,14 @@ namespace GraphQL\Language;
 use GraphQL\Language\AST\ArgumentNode;
 use GraphQL\Language\AST\DirectiveDefinitionNode;
 use GraphQL\Language\AST\EnumTypeDefinitionNode;
+use GraphQL\Language\AST\EnumTypeExtensionNode;
 use GraphQL\Language\AST\EnumValueDefinitionNode;
 use GraphQL\Language\AST\FieldDefinitionNode;
 use GraphQL\Language\AST\InputObjectTypeDefinitionNode;
+use GraphQL\Language\AST\InputObjectTypeExtensionNode;
 use GraphQL\Language\AST\InputValueDefinitionNode;
 use GraphQL\Language\AST\InterfaceTypeDefinitionNode;
+use GraphQL\Language\AST\InterfaceTypeExtensionNode;
 use GraphQL\Language\AST\ListValueNode;
 use GraphQL\Language\AST\BooleanValueNode;
 use GraphQL\Language\AST\DirectiveNode;
@@ -33,6 +36,7 @@ use GraphQL\Language\AST\ObjectValueNode;
 use GraphQL\Language\AST\OperationDefinitionNode;
 use GraphQL\Language\AST\OperationTypeDefinitionNode;
 use GraphQL\Language\AST\ScalarTypeDefinitionNode;
+use GraphQL\Language\AST\ScalarTypeExtensionNode;
 use GraphQL\Language\AST\SchemaDefinitionNode;
 use GraphQL\Language\AST\SelectionSetNode;
 use GraphQL\Language\AST\StringValueNode;
@@ -40,9 +44,11 @@ use GraphQL\Language\AST\ObjectTypeExtensionNode;
 use GraphQL\Language\AST\TypeExtensionNode;
 use GraphQL\Language\AST\TypeSystemDefinitionNode;
 use GraphQL\Language\AST\UnionTypeDefinitionNode;
+use GraphQL\Language\AST\UnionTypeExtensionNode;
 use GraphQL\Language\AST\VariableNode;
 use GraphQL\Language\AST\VariableDefinitionNode;
 use GraphQL\Error\SyntaxError;
+use GraphQL\Type\TypeKind;
 
 /**
  * Parses string containing GraphQL query or [type definition](type-system/type-language.md) to Abstract Syntax Tree.
@@ -1002,7 +1008,7 @@ class Parser
         $name = $this->parseName();
         $interfaces = $this->parseImplementsInterfaces();
         $directives = $this->parseDirectives(true);
-        $fields = $this->parseFieldDefinitions();
+        $fields = $this->parseFieldsDefinition();
 
         return new ObjectTypeDefinitionNode([
             'name' => $name,
@@ -1033,13 +1039,15 @@ class Parser
      * @return FieldDefinitionNode[]|NodeList
      * @throws SyntaxError
      */
-    function parseFieldDefinitions()
+    function parseFieldsDefinition()
     {
-        return $this->many(
-            Token::BRACE_L,
-            [$this, 'parseFieldDefinition'],
-            Token::BRACE_R
-        );
+        return $this->peek(Token::BRACE_L)
+            ? $this->many(
+                Token::BRACE_L,
+                [$this, 'parseFieldDefinition'],
+                Token::BRACE_R
+            )
+            : new NodeList([]);
     }
 
     /**
@@ -1114,7 +1122,7 @@ class Parser
         $this->expectKeyword('interface');
         $name = $this->parseName();
         $directives = $this->parseDirectives(true);
-        $fields = $this->parseFieldDefinitions();
+        $fields = $this->parseFieldsDefinition();
 
         return new InterfaceTypeDefinitionNode([
             'name' => $name,
@@ -1136,8 +1144,7 @@ class Parser
         $this->expectKeyword('union');
         $name = $this->parseName();
         $directives = $this->parseDirectives(true);
-        $this->expect(Token::EQUALS);
-        $types = $this->parseUnionMembers();
+        $types = $this->parseMemberTypesDefinition();
 
         return new UnionTypeDefinitionNode([
             'name' => $name,
@@ -1149,22 +1156,23 @@ class Parser
     }
 
     /**
-     * UnionMembers :
+     * MemberTypes :
      *   - `|`? NamedType
-     *   - UnionMembers | NamedType
+     *   - MemberTypes | NamedType
      *
      * @return NamedTypeNode[]
      */
-    function parseUnionMembers()
+    function parseMemberTypesDefinition()
     {
-        // Optional leading pipe
-        $this->skip(Token::PIPE);
-        $members = [];
-
-        do {
-            $members[] = $this->parseNamedType();
-        } while ($this->skip(Token::PIPE));
-        return $members;
+        $types = [];
+        if ($this->skip(Token::EQUALS)) {
+            // Optional leading pipe
+            $this->skip(Token::PIPE);
+            do {
+                $types[] = $this->parseNamedType();
+            } while ($this->skip(Token::PIPE));
+        }
+        return $types;
     }
 
     /**
@@ -1178,11 +1186,7 @@ class Parser
         $this->expectKeyword('enum');
         $name = $this->parseName();
         $directives = $this->parseDirectives(true);
-        $values = $this->many(
-            Token::BRACE_L,
-            [$this, 'parseEnumValueDefinition'],
-            Token::BRACE_R
-        );
+        $values = $this->parseEnumValuesDefinition();
 
         return new EnumTypeDefinitionNode([
             'name' => $name,
@@ -1193,6 +1197,21 @@ class Parser
         ]);
     }
 
+    /**
+     * @return EnumValueDefinitionNode[]|NodeList
+     * @throws SyntaxError
+     */
+    function parseEnumValuesDefinition()
+    {
+        return $this->peek(Token::BRACE_L)
+            ? $this->many(
+                Token::BRACE_L,
+                [$this, 'parseEnumValueDefinition'],
+                Token::BRACE_R
+              )
+            : new NodeList([]);
+    }
+
     /**
      * @return EnumValueDefinitionNode
      * @throws SyntaxError
@@ -1223,11 +1242,7 @@ class Parser
         $this->expectKeyword('input');
         $name = $this->parseName();
         $directives = $this->parseDirectives(true);
-        $fields = $this->many(
-            Token::BRACE_L,
-            [$this, 'parseInputValueDef'],
-            Token::BRACE_R
-        );
+        $fields = $this->parseInputFieldsDefinition();
 
         return new InputObjectTypeDefinitionNode([
             'name' => $name,
@@ -1239,6 +1254,28 @@ class Parser
     }
 
     /**
+     * @return InputValueDefinitionNode[]|NodeList
+     * @throws SyntaxError
+     */
+    function parseInputFieldsDefinition() {
+        return $this->peek(Token::BRACE_L)
+            ? $this->many(
+                Token::BRACE_L,
+                [$this, 'parseInputValueDef'],
+                Token::BRACE_R
+            )
+            : new NodeList([]);
+    }
+
+    /**
+     * TypeExtension :
+     *   - ScalarTypeExtension
+     *   - ObjectTypeExtension
+     *   - InterfaceTypeExtension
+     *   - UnionTypeExtension
+     *   - EnumTypeExtension
+     *   - InputObjectTypeDefinition
+     *
      * @return TypeExtensionNode
      * @throws SyntaxError
      */
@@ -1248,14 +1285,45 @@ class Parser
 
         if ($keywordToken->kind === Token::NAME) {
             switch ($keywordToken->value) {
+                case 'scalar':
+                    return $this->parseScalarTypeExtension();
                 case 'type':
                     return $this->parseObjectTypeExtension();
+                case 'interface':
+                    return $this->parseInterfaceTypeExtension();
+                case 'union':
+                    return $this->parseUnionTypeExtension();
+                case 'enum':
+                    return $this->parseEnumTypeExtension();
+                case 'input':
+                    return $this->parseInputObjectTypeExtension();
             }
         }
 
         throw $this->unexpected($keywordToken);
     }
 
+    /**
+     * @return ScalarTypeExtensionNode
+     * @throws SyntaxError
+     */
+    function parseScalarTypeExtension() {
+        $start = $this->lexer->token;
+        $this->expectKeyword('extend');
+        $this->expectKeyword('scalar');
+        $name = $this->parseName();
+        $directives = $this->parseDirectives(true);
+        if (count($directives) === 0) {
+            throw $this->unexpected();
+        }
+
+        return new ScalarTypeExtensionNode([
+            'name' => $name,
+            'directives' => $directives,
+            'loc' => $this->loc($start)
+        ]);
+    }
+
     /**
      * @return ObjectTypeExtensionNode
      * @throws SyntaxError
@@ -1267,9 +1335,7 @@ class Parser
         $name = $this->parseName();
         $interfaces = $this->parseImplementsInterfaces();
         $directives = $this->parseDirectives(true);
-        $fields = $this->peek(Token::BRACE_L)
-            ? $this->parseFieldDefinitions()
-            : [];
+        $fields = $this->parseFieldsDefinition();
 
         if (
             count($interfaces) === 0 &&
@@ -1288,6 +1354,110 @@ class Parser
         ]);
     }
 
+    /**
+     * @return InterfaceTypeExtensionNode
+     * @throws SyntaxError
+     */
+    function parseInterfaceTypeExtension() {
+        $start = $this->lexer->token;
+        $this->expectKeyword('extend');
+        $this->expectKeyword('interface');
+        $name = $this->parseName();
+        $directives = $this->parseDirectives(true);
+        $fields = $this->parseFieldsDefinition();
+        if (
+            count($directives) === 0 &&
+            count($fields) === 0
+        ) {
+            throw $this->unexpected();
+        }
+
+        return new InterfaceTypeExtensionNode([
+            'name' => $name,
+            'directives' => $directives,
+            'fields' => $fields,
+            'loc' => $this->loc($start)
+        ]);
+    }
+
+    /**
+     * @return UnionTypeExtensionNode
+     * @throws SyntaxError
+     */
+    function parseUnionTypeExtension() {
+        $start = $this->lexer->token;
+        $this->expectKeyword('extend');
+        $this->expectKeyword('union');
+        $name = $this->parseName();
+        $directives = $this->parseDirectives(true);
+        $types = $this->parseMemberTypesDefinition();
+        if (
+            count($directives) === 0 &&
+            count($types) === 0
+        ) {
+            throw $this->unexpected();
+        }
+
+        return new UnionTypeExtensionNode([
+            'name' => $name,
+            'directives' => $directives,
+            'types' => $types,
+            'loc' => $this->loc($start)
+        ]);
+    }
+
+    /**
+     * @return EnumTypeExtensionNode
+     * @throws SyntaxError
+     */
+    function parseEnumTypeExtension() {
+        $start = $this->lexer->token;
+        $this->expectKeyword('extend');
+        $this->expectKeyword('enum');
+        $name = $this->parseName();
+        $directives = $this->parseDirectives(true);
+        $values = $this->parseEnumValuesDefinition();
+        if (
+            count($directives) === 0 &&
+            count($values) === 0
+        ) {
+            throw $this->unexpected();
+        }
+
+        return new EnumTypeExtensionNode([
+            'name' => $name,
+            'directives' => $directives,
+            'values' => $values,
+            'loc' => $this->loc($start)
+        ]);
+    }
+
+    /**
+     * @return InputObjectTypeExtensionNode
+     * @throws SyntaxError
+     */
+    function parseInputObjectTypeExtension() {
+        $start = $this->lexer->token;
+        $this->expectKeyword('extend');
+        $this->expectKeyword('input');
+        $name = $this->parseName();
+        $directives = $this->parseDirectives(true);
+        $fields = $this->parseInputFieldsDefinition();
+        if (
+            count($directives) === 0 &&
+            count($fields) === 0
+        ) {
+            throw $this->unexpected();
+        }
+
+        return new InputObjectTypeExtensionNode([
+            'name' => $name,
+            'directives' => $directives,
+            'fields' => $fields,
+            'loc' => $this->loc($start)
+        ]);
+    }
+
     /**
      * DirectiveDefinition :
      *   - directive @ Name ArgumentsDefinition? on DirectiveLocations
diff --git a/src/Language/Printer.php b/src/Language/Printer.php
index b835b4d..7c123a3 100644
--- a/src/Language/Printer.php
+++ b/src/Language/Printer.php
@@ -4,11 +4,14 @@ namespace GraphQL\Language;
 use GraphQL\Language\AST\ArgumentNode;
 use GraphQL\Language\AST\DirectiveDefinitionNode;
 use GraphQL\Language\AST\EnumTypeDefinitionNode;
+use GraphQL\Language\AST\EnumTypeExtensionNode;
 use GraphQL\Language\AST\EnumValueDefinitionNode;
 use GraphQL\Language\AST\FieldDefinitionNode;
 use GraphQL\Language\AST\InputObjectTypeDefinitionNode;
+use GraphQL\Language\AST\InputObjectTypeExtensionNode;
 use GraphQL\Language\AST\InputValueDefinitionNode;
 use GraphQL\Language\AST\InterfaceTypeDefinitionNode;
+use GraphQL\Language\AST\InterfaceTypeExtensionNode;
 use GraphQL\Language\AST\ListValueNode;
 use GraphQL\Language\AST\BooleanValueNode;
 use GraphQL\Language\AST\DirectiveNode;
@@ -32,11 +35,13 @@ use GraphQL\Language\AST\ObjectValueNode;
 use GraphQL\Language\AST\OperationDefinitionNode;
 use GraphQL\Language\AST\OperationTypeDefinitionNode;
 use GraphQL\Language\AST\ScalarTypeDefinitionNode;
+use GraphQL\Language\AST\ScalarTypeExtensionNode;
 use GraphQL\Language\AST\SchemaDefinitionNode;
 use GraphQL\Language\AST\SelectionSetNode;
 use GraphQL\Language\AST\StringValueNode;
 use GraphQL\Language\AST\ObjectTypeExtensionNode;
 use GraphQL\Language\AST\UnionTypeDefinitionNode;
+use GraphQL\Language\AST\UnionTypeExtensionNode;
 use GraphQL\Language\AST\VariableDefinitionNode;
 use GraphQL\Utils\Utils;
 
@@ -246,7 +251,9 @@ class Printer
                             'union',
                             $def->name,
                             $this->join($def->directives, ' '),
-                            '= ' . $this->join($def->types, ' | ')
+                            $def->types
+                                ? '= ' . $this->join($def->types, ' | ')
+                                : ''
                         ], ' ')
                     ], "\n");
                 },
@@ -278,6 +285,13 @@ class Printer
                         ], ' ')
                     ], "\n");
                 },
+                NodeKind::SCALAR_TYPE_EXTENSION => function(ScalarTypeExtensionNode $def) {
+                    return $this->join([
+                        'extend scalar',
+                        $def->name,
+                        $this->join($def->directives, ' '),
+                    ], ' ');
+                },
                 NodeKind::OBJECT_TYPE_EXTENSION => function(ObjectTypeExtensionNode $def) {
                     return $this->join([
                         'extend type',
@@ -287,6 +301,40 @@ class Printer
                         $this->block($def->fields),
                     ], ' ');
                 },
+                NodeKind::INTERFACE_TYPE_EXTENSION => function(InterfaceTypeExtensionNode $def) {
+                    return $this->join([
+                        'extend interface',
+                        $def->name,
+                        $this->join($def->directives, ' '),
+                        $this->block($def->fields),
+                    ], ' ');
+                },
+                NodeKind::UNION_TYPE_EXTENSION => function(UnionTypeExtensionNode $def) {
+                    return $this->join([
+                        'extend union',
+                        $def->name,
+                        $this->join($def->directives, ' '),
+                        $def->types
+                            ? '= ' . $this->join($def->types, ' | ')
+                            : ''
+                    ], ' ');
+                },
+                NodeKind::ENUM_TYPE_EXTENSION => function(EnumTypeExtensionNode $def) {
+                    return $this->join([
+                        'extend enum',
+                        $def->name,
+                        $this->join($def->directives, ' '),
+                        $this->block($def->values),
+                    ], ' ');
+                },
+                NodeKind::INPUT_OBJECT_TYPE_EXTENSION => function(InputObjectTypeExtensionNode $def) {
+                    return $this->join([
+                        'extend input',
+                        $def->name,
+                        $this->join($def->directives, ' '),
+                        $this->block($def->fields),
+                    ], ' ');
+                },
                 NodeKind::DIRECTIVE_DEFINITION => function(DirectiveDefinitionNode $def) {
                     return $this->join([
                         $def->description,
diff --git a/src/Language/Visitor.php b/src/Language/Visitor.php
index fb149cd..fc0a1e7 100644
--- a/src/Language/Visitor.php
+++ b/src/Language/Visitor.php
@@ -142,7 +142,14 @@ class Visitor
         NodeKind::ENUM_TYPE_DEFINITION => ['description', 'name', 'directives', 'values'],
         NodeKind::ENUM_VALUE_DEFINITION => ['description', 'name', 'directives'],
         NodeKind::INPUT_OBJECT_TYPE_DEFINITION => ['description', 'name', 'directives', 'fields'],
-        NodeKind::OBJECT_TYPE_EXTENSION => [ 'name', 'interfaces', 'directives', 'fields' ],
+
+        NodeKind::SCALAR_TYPE_EXTENSION => ['name', 'directives'],
+        NodeKind::OBJECT_TYPE_EXTENSION => ['name', 'interfaces', 'directives', 'fields'],
+        NodeKind::INTERFACE_TYPE_EXTENSION => ['name', 'directives', 'fields'],
+        NodeKind::UNION_TYPE_EXTENSION => ['name', 'directives', 'types'],
+        NodeKind::ENUM_TYPE_EXTENSION => ['name', 'directives', 'values'],
+        NodeKind::INPUT_OBJECT_TYPE_EXTENSION => ['name', 'directives', 'fields'],
+
         NodeKind::DIRECTIVE_DEFINITION => ['description', 'name', 'arguments', 'locations']
     ];
 
diff --git a/src/Utils/ASTDefinitionBuilder.php b/src/Utils/ASTDefinitionBuilder.php
index d6d7f7a..d38310b 100644
--- a/src/Utils/ASTDefinitionBuilder.php
+++ b/src/Utils/ASTDefinitionBuilder.php
@@ -251,15 +251,17 @@ class ASTDefinitionBuilder
 
     private function makeFieldDefMap($def)
     {
-        return Utils::keyValMap(
-            $def->fields,
-            function ($field) {
-                return $field->name->value;
-            },
-            function ($field) {
-                return $this->buildField($field);
-            }
-        );
+        return $def->fields
+            ? Utils::keyValMap(
+                $def->fields,
+                function ($field) {
+                    return $field->name->value;
+                },
+                function ($field) {
+                    return $this->buildField($field);
+                }
+            )
+            : [];
     }
 
     private function makeImplementedInterfaces(ObjectTypeDefinitionNode $def)
@@ -313,20 +315,22 @@ class ASTDefinitionBuilder
         return new EnumType([
             'name' => $def->name->value,
             'description' => $this->getDescription($def),
+            'values' => $def->values
+                ? Utils::keyValMap(
+                    $def->values,
+                    function ($enumValue) {
+                        return $enumValue->name->value;
+                    },
+                    function ($enumValue) {
+                        return [
+                            'description' => $this->getDescription($enumValue),
+                            'deprecationReason' => $this->getDeprecationReason($enumValue),
+                            'astNode' => $enumValue
+                        ];
+                    }
+                )
+                : [],
             'astNode' => $def,
-            'values' => Utils::keyValMap(
-                $def->values,
-                function ($enumValue) {
-                    return $enumValue->name->value;
-                },
-                function ($enumValue) {
-                    return [
-                        'description' => $this->getDescription($enumValue),
-                        'deprecationReason' => $this->getDeprecationReason($enumValue),
-                        'astNode' => $enumValue
-                    ];
-                }
-            )
         ]);
     }
 
@@ -335,10 +339,12 @@ class ASTDefinitionBuilder
         return new UnionType([
             'name' => $def->name->value,
             'description' => $this->getDescription($def),
-            'types' => Utils::map($def->types, function ($typeNode) {
-                return $this->buildObjectType($typeNode);
-            }),
-            'astNode' => $def
+            'types' => $def->types
+                ? Utils::map($def->types, function ($typeNode) {
+                    return $this->buildObjectType($typeNode);
+                }):
+                [],
+            'astNode' => $def,
         ]);
     }
 
@@ -360,7 +366,9 @@ class ASTDefinitionBuilder
             'name' => $def->name->value,
             'description' => $this->getDescription($def),
             'fields' => function () use ($def) {
-                return $this->makeInputValues($def->fields);
+                return $def->fields
+                    ? $this->makeInputValues($def->fields)
+                    : [];
             },
             'astNode' => $def,
         ]);
diff --git a/src/Validator/Rules/KnownDirectives.php b/src/Validator/Rules/KnownDirectives.php
index 9d48aa5..4ec3a01 100644
--- a/src/Validator/Rules/KnownDirectives.php
+++ b/src/Validator/Rules/KnownDirectives.php
@@ -67,20 +67,38 @@ class KnownDirectives extends AbstractValidationRule
                     case 'subscription': return DirectiveLocation::SUBSCRIPTION;
                 }
                 break;
-            case NodeKind::FIELD: return DirectiveLocation::FIELD;
-            case NodeKind::FRAGMENT_SPREAD: return DirectiveLocation::FRAGMENT_SPREAD;
-            case NodeKind::INLINE_FRAGMENT: return DirectiveLocation::INLINE_FRAGMENT;
-            case NodeKind::FRAGMENT_DEFINITION: return DirectiveLocation::FRAGMENT_DEFINITION;
-            case NodeKind::SCHEMA_DEFINITION: return DirectiveLocation::SCHEMA;
-            case NodeKind::SCALAR_TYPE_DEFINITION: return DirectiveLocation::SCALAR;
+            case NodeKind::FIELD:
+                return DirectiveLocation::FIELD;
+            case NodeKind::FRAGMENT_SPREAD:
+                return DirectiveLocation::FRAGMENT_SPREAD;
+            case NodeKind::INLINE_FRAGMENT:
+                return DirectiveLocation::INLINE_FRAGMENT;
+            case NodeKind::FRAGMENT_DEFINITION:
+                return DirectiveLocation::FRAGMENT_DEFINITION;
+            case NodeKind::SCHEMA_DEFINITION:
+                return DirectiveLocation::SCHEMA;
+            case NodeKind::SCALAR_TYPE_DEFINITION:
+            case NodeKind::SCALAR_TYPE_EXTENSION:
+                return DirectiveLocation::SCALAR;
             case NodeKind::OBJECT_TYPE_DEFINITION:
-            case NodeKind::OBJECT_TYPE_EXTENSION: return DirectiveLocation::OBJECT;
-            case NodeKind::FIELD_DEFINITION: return DirectiveLocation::FIELD_DEFINITION;
-            case NodeKind::INTERFACE_TYPE_DEFINITION: return DirectiveLocation::IFACE;
-            case NodeKind::UNION_TYPE_DEFINITION: return DirectiveLocation::UNION;
-            case NodeKind::ENUM_TYPE_DEFINITION: return DirectiveLocation::ENUM;
-            case NodeKind::ENUM_VALUE_DEFINITION: return DirectiveLocation::ENUM_VALUE;
-            case NodeKind::INPUT_OBJECT_TYPE_DEFINITION: return DirectiveLocation::INPUT_OBJECT;
+            case NodeKind::OBJECT_TYPE_EXTENSION:
+                return DirectiveLocation::OBJECT;
+            case NodeKind::FIELD_DEFINITION:
+                return DirectiveLocation::FIELD_DEFINITION;
+            case NodeKind::INTERFACE_TYPE_DEFINITION:
+            case NodeKind::INTERFACE_TYPE_EXTENSION:
+                return DirectiveLocation::IFACE;
+            case NodeKind::UNION_TYPE_DEFINITION:
+            case NodeKind::UNION_TYPE_EXTENSION:
+                return DirectiveLocation::UNION;
+            case NodeKind::ENUM_TYPE_DEFINITION:
+            case NodeKind::ENUM_TYPE_EXTENSION:
+                return DirectiveLocation::ENUM;
+            case NodeKind::ENUM_VALUE_DEFINITION:
+                return DirectiveLocation::ENUM_VALUE;
+            case NodeKind::INPUT_OBJECT_TYPE_DEFINITION:
+            case NodeKind::INPUT_OBJECT_TYPE_EXTENSION:
+                return DirectiveLocation::INPUT_OBJECT;
             case NodeKind::INPUT_VALUE_DEFINITION:
                 $parentNode = $ancestors[count($ancestors) - 3];
                 return $parentNode instanceof InputObjectTypeDefinitionNode
diff --git a/tests/Language/SchemaPrinterTest.php b/tests/Language/SchemaPrinterTest.php
index 3141f06..1e1b897 100644
--- a/tests/Language/SchemaPrinterTest.php
+++ b/tests/Language/SchemaPrinterTest.php
@@ -74,6 +74,14 @@ type AnnotatedObject @onObject(arg: "value") {
   annotatedField(arg: Type = "default" @onArg): Type @onField
 }
 
+type UndefinedType
+
+extend type Foo {
+  seven(argument: [String]): Type
+}
+
+extend type Foo @onType
+
 interface Bar {
   one: Type
   four(argument: String = "string"): String
@@ -83,16 +91,32 @@ interface AnnotatedInterface @onInterface {
   annotatedField(arg: Type @onArg): Type @onField
 }
 
+interface UndefinedInterface
+
+extend interface Bar {
+  two(argument: InputType!): Type
+}
+
+extend interface Bar @onInterface
+
 union Feed = Story | Article | Advert
 
 union AnnotatedUnion @onUnion = A | B
 
 union AnnotatedUnionTwo @onUnion = A | B
 
+union UndefinedUnion
+
+extend union Feed = Photo | Video
+
+extend union Feed @onUnion
+
 scalar CustomScalar
 
 scalar AnnotatedScalar @onScalar
 
+extend scalar CustomScalar @onScalar
+
 enum Site {
   DESKTOP
   MOBILE
@@ -103,20 +127,30 @@ enum AnnotatedEnum @onEnum {
   OTHER_VALUE
 }
 
+enum UndefinedEnum
+
+extend enum Site {
+  VR
+}
+
+extend enum Site @onEnum
+
 input InputType {
   key: String!
   answer: Int = 42
 }
 
-input AnnotatedInput @onInputObjectType {
+input AnnotatedInput @onInputObject {
   annotatedField: Type @onField
 }
 
-extend type Foo {
-  seven(argument: [String]): Type
+input UndefinedInput
+
+extend input InputType {
+  other: Float = 1.23e4
 }
 
-extend type Foo @onType
+extend input InputType @onInputObject
 
 directive @skip(if: Boolean!) on FIELD | FRAGMENT_SPREAD | INLINE_FRAGMENT
 
diff --git a/tests/Language/schema-kitchen-sink.graphql b/tests/Language/schema-kitchen-sink.graphql
index 016707d..ae1a3e5 100644
--- a/tests/Language/schema-kitchen-sink.graphql
+++ b/tests/Language/schema-kitchen-sink.graphql
@@ -13,20 +13,28 @@ This is a description
 of the `Foo` type.
 """
 type Foo implements Bar {
-  one: Type
-  two(argument: InputType!): Type
-  three(argument: InputType, other: String): Int
-  four(argument: String = "string"): String
-  five(argument: [String] = ["string", "string"]): String
-  six(argument: InputType = {key: "value"}): Type
-  seven(argument: Int = null): Type
+one: Type
+two(argument: InputType!): Type
+three(argument: InputType, other: String): Int
+four(argument: String = "string"): String
+five(argument: [String] = ["string", "string"]): String
+six(argument: InputType = {key: "value"}): Type
+seven(argument: Int = null): Type
 }
 
 type AnnotatedObject @onObject(arg: "value") {
   annotatedField(arg: Type = "default" @onArg): Type @onField
 }
 
-interface Bar {
+type UndefinedType
+
+  extend type Foo {
+  seven(argument: [String]): Type
+}
+
+extend type Foo @onType
+
+  interface Bar {
   one: Type
   four(argument: String = "string"): String
 }
@@ -35,49 +43,75 @@ interface AnnotatedInterface @onInterface {
   annotatedField(arg: Type @onArg): Type @onField
 }
 
+interface UndefinedInterface
+
+  extend interface Bar {
+  two(argument: InputType!): Type
+}
+
+extend interface Bar @onInterface
+
 union Feed = Story | Article | Advert
 
 union AnnotatedUnion @onUnion = A | B
 
 union AnnotatedUnionTwo @onUnion = | A | B
 
+union UndefinedUnion
+
+extend union Feed = Photo | Video
+
+extend union Feed @onUnion
+
 scalar CustomScalar
 
 scalar AnnotatedScalar @onScalar
 
+extend scalar CustomScalar @onScalar
+
 enum Site {
-  DESKTOP
-  MOBILE
+DESKTOP
+MOBILE
 }
 
 enum AnnotatedEnum @onEnum {
-  ANNOTATED_VALUE @onEnumValue
-  OTHER_VALUE
+ANNOTATED_VALUE @onEnumValue
+OTHER_VALUE
 }
 
+enum UndefinedEnum
+
+extend enum Site {
+VR
+}
+
+extend enum Site @onEnum
+
 input InputType {
-  key: String!
-  answer: Int = 42
+key: String!
+answer: Int = 42
 }
 
-input AnnotatedInput @onInputObjectType {
-  annotatedField: Type @onField
+input AnnotatedInput @onInputObject {
+annotatedField: Type @onField
 }
 
-extend type Foo {
-  seven(argument: [String]): Type
+input UndefinedInput
+
+extend input InputType {
+other: Float = 1.23e4
 }
 
-extend type Foo @onType
+extend input InputType @onInputObject
 
 directive @skip(if: Boolean!) on FIELD | FRAGMENT_SPREAD | INLINE_FRAGMENT
 
 directive @include(if: Boolean!)
-  on FIELD
-   | FRAGMENT_SPREAD
-   | INLINE_FRAGMENT
+on FIELD
+| FRAGMENT_SPREAD
+| INLINE_FRAGMENT
 
 directive @include2(if: Boolean!) on
-  | FIELD
-  | FRAGMENT_SPREAD
-  | INLINE_FRAGMENT
+| FIELD
+| FRAGMENT_SPREAD
+| INLINE_FRAGMENT
diff --git a/tests/Validator/KnownDirectivesTest.php b/tests/Validator/KnownDirectivesTest.php
index 3e7998e..9374b73 100644
--- a/tests/Validator/KnownDirectivesTest.php
+++ b/tests/Validator/KnownDirectivesTest.php
@@ -139,20 +139,30 @@ class KnownDirectivesTest extends TestCase
         extend type MyObj @onObject
 
         scalar MyScalar @onScalar
+        
+        extend scalar MyScalar @onScalar
 
         interface MyInterface @onInterface {
           myField(myArg: Int @onArgumentDefinition): String @onFieldDefinition
         }
+        
+        extend interface MyInterface @onInterface
 
         union MyUnion @onUnion = MyObj | Other
+        
+        extend union MyUnion @onUnion
 
         enum MyEnum @onEnum {
           MY_VALUE @onEnumValue
         }
+        
+        extend enum MyEnum @onEnum
 
         input MyInput @onInputObject {
           myField: Int @onInputFieldDefinition
         }
+        
+        extend input MyInput @onInputObject
 
         schema @onSchema {
           query: MyQuery