diff --git a/src/Language/AST/Definition.php b/src/Language/AST/Definition.php
index c826e2e..d7dec6e 100644
--- a/src/Language/AST/Definition.php
+++ b/src/Language/AST/Definition.php
@@ -6,5 +6,6 @@ interface Definition
 /**
  * export type Definition = OperationDefinition
  *                        | FragmentDefinition
+ *                        | TypeSystemDefinition // experimental non-spec addition.
  */
 }
diff --git a/src/Language/AST/DirectiveDefinition.php b/src/Language/AST/DirectiveDefinition.php
new file mode 100644
index 0000000..81cd696
--- /dev/null
+++ b/src/Language/AST/DirectiveDefinition.php
@@ -0,0 +1,25 @@
+<?php
+namespace GraphQL\Language\AST;
+
+class DirectiveDefinition extends Node implements TypeSystemDefinition
+{
+    /**
+     * @var string
+     */
+    public $kind = Node::DIRECTIVE_DEFINITION;
+
+    /**
+     * @var Name
+     */
+    public $name;
+
+    /**
+     * @var Argument[]
+     */
+    public $arguments;
+
+    /**
+     * @var Name[]
+     */
+    public $locations;
+}
diff --git a/src/Language/AST/EnumTypeDefinition.php b/src/Language/AST/EnumTypeDefinition.php
new file mode 100644
index 0000000..d1c653e
--- /dev/null
+++ b/src/Language/AST/EnumTypeDefinition.php
@@ -0,0 +1,20 @@
+<?php
+namespace GraphQL\Language\AST;
+
+class EnumTypeDefinition extends Node implements TypeDefinition
+{
+    /**
+     * @var string
+     */
+    public $kind = self::UNION_TYPE_DEFINITION;
+
+    /**
+     * @var Name
+     */
+    public $name;
+
+    /**
+     * @var EnumValueDefinition[]
+     */
+    public $values;
+}
diff --git a/src/Language/AST/EnumValueDefinition.php b/src/Language/AST/EnumValueDefinition.php
new file mode 100644
index 0000000..c2b0a49
--- /dev/null
+++ b/src/Language/AST/EnumValueDefinition.php
@@ -0,0 +1,15 @@
+<?php
+namespace GraphQL\Language\AST;
+
+class EnumValueDefinition extends Node
+{
+    /**
+     * @var string
+     */
+    public $kind = Node::ENUM_VALUE_DEFINITION;
+
+    /**
+     * @var Name
+     */
+    public $name;
+}
diff --git a/src/Language/AST/Field.php b/src/Language/AST/Field.php
index 6535b0b..7a2f997 100644
--- a/src/Language/AST/Field.php
+++ b/src/Language/AST/Field.php
@@ -1,7 +1,7 @@
 <?php
 namespace GraphQL\Language\AST;
 
-class Field extends Node
+class Field extends Node implements Selection
 {
     public $kind = Node::FIELD;
 
diff --git a/src/Language/AST/FieldDefinition.php b/src/Language/AST/FieldDefinition.php
new file mode 100644
index 0000000..801e8d0
--- /dev/null
+++ b/src/Language/AST/FieldDefinition.php
@@ -0,0 +1,25 @@
+<?php
+namespace GraphQL\Language\AST;
+
+class FieldDefinition extends Node
+{
+    /**
+     * @var string
+     */
+    public $kind = Node::FIELD_DEFINITION;
+
+    /**
+     * @var Name
+     */
+    public $name;
+
+    /**
+     * @var InputValueDefinition[]
+     */
+    public $arguments;
+
+    /**
+     * @var Type
+     */
+    public $type;
+}
diff --git a/src/Language/AST/FragmentSpread.php b/src/Language/AST/FragmentSpread.php
index a989bc2..ccb2798 100644
--- a/src/Language/AST/FragmentSpread.php
+++ b/src/Language/AST/FragmentSpread.php
@@ -1,7 +1,7 @@
 <?php
 namespace GraphQL\Language\AST;
 
-class FragmentSpread extends Node
+class FragmentSpread extends Node implements Selection
 {
     public $kind = Node::FRAGMENT_SPREAD;
 
diff --git a/src/Language/AST/InlineFragment.php b/src/Language/AST/InlineFragment.php
index 46343f4..9691f31 100644
--- a/src/Language/AST/InlineFragment.php
+++ b/src/Language/AST/InlineFragment.php
@@ -1,7 +1,7 @@
 <?php
 namespace GraphQL\Language\AST;
 
-class InlineFragment extends Node
+class InlineFragment extends Node implements Selection
 {
     public $kind = Node::INLINE_FRAGMENT;
 
diff --git a/src/Language/AST/InputObjectTypeDefinition.php b/src/Language/AST/InputObjectTypeDefinition.php
new file mode 100644
index 0000000..af4d67c
--- /dev/null
+++ b/src/Language/AST/InputObjectTypeDefinition.php
@@ -0,0 +1,20 @@
+<?php
+namespace GraphQL\Language\AST;
+
+class InputObjectTypeDefinition extends Node implements TypeDefinition
+{
+    /**
+     * @var string
+     */
+    public $kind = Node::INPUT_OBJECT_TYPE_DEFINITION;
+
+    /**
+     * @var Name
+     */
+    public $name;
+
+    /**
+     * @var InputValueDefinition[]
+     */
+    public $fields;
+}
diff --git a/src/Language/AST/InputValueDefinition.php b/src/Language/AST/InputValueDefinition.php
new file mode 100644
index 0000000..bbf7b72
--- /dev/null
+++ b/src/Language/AST/InputValueDefinition.php
@@ -0,0 +1,25 @@
+<?php
+namespace GraphQL\Language\AST;
+
+class InputValueDefinition extends Node
+{
+    /**
+     * @var string
+     */
+    public $kind = Node::INPUT_VALUE_DEFINITION;
+
+    /**
+     * @var Name
+     */
+    public $name;
+
+    /**
+     * @var Type
+     */
+    public $type;
+
+    /**
+     * @var Value
+     */
+    public $defaultValue;
+}
\ No newline at end of file
diff --git a/src/Language/AST/InterfaceTypeDefinition.php b/src/Language/AST/InterfaceTypeDefinition.php
new file mode 100644
index 0000000..da0f402
--- /dev/null
+++ b/src/Language/AST/InterfaceTypeDefinition.php
@@ -0,0 +1,20 @@
+<?php
+namespace GraphQL\Language\AST;
+
+class InterfaceTypeDefinition extends Node implements TypeDefinition
+{
+    /**
+     * @var string
+     */
+    public $kind = Node::INTERFACE_TYPE_DEFINITION;
+
+    /**
+     * @var Name
+     */
+    public $name;
+
+    /**
+     * @var FieldDefinition[]
+     */
+    public $fields = [];
+}
\ No newline at end of file
diff --git a/src/Language/AST/Node.php b/src/Language/AST/Node.php
index d4d061e..ce7c06f 100644
--- a/src/Language/AST/Node.php
+++ b/src/Language/AST/Node.php
@@ -46,7 +46,31 @@ abstract class Node
     const LIST_TYPE = 'ListType';
     const NON_NULL_TYPE = 'NonNullType';
 
-    
+    // Type System Definitions
+
+    const SCHEMA_DEFINITION = 'SchemaDefinition';
+    const OPERATION_TYPE_DEFINITION = 'OperationTypeDefinition';
+
+    // Type Definitions
+
+    const SCALAR_TYPE_DEFINITION = 'ScalarTypeDefinition';
+    const OBJECT_TYPE_DEFINITION = 'ObjectTypeDefinition';
+    const FIELD_DEFINITION = 'FieldDefinition';
+    const INPUT_VALUE_DEFINITION = 'InputValueDefinition';
+    const INTERFACE_TYPE_DEFINITION = 'InterfaceTypeDefinition';
+    const UNION_TYPE_DEFINITION = 'UnionTypeDefinition';
+    const ENUM_TYPE_DEFINITION = 'EnumTypeDefinition';
+    const ENUM_VALUE_DEFINITION = 'EnumValueDefinition';
+    const INPUT_OBJECT_TYPE_DEFINITION = 'InputObjectTypeDefinition';
+
+    // Type Extensions
+
+    const TYPE_EXTENSION_DEFINITION = 'TypeExtensionDefinition';
+
+    // Directive Definitions
+
+    const DIRECTIVE_DEFINITION = 'DirectiveDefinition';
+
     /**
         type Node = Name
     | Document
diff --git a/src/Language/AST/ObjectTypeDefinition.php b/src/Language/AST/ObjectTypeDefinition.php
new file mode 100644
index 0000000..fb4e19b
--- /dev/null
+++ b/src/Language/AST/ObjectTypeDefinition.php
@@ -0,0 +1,25 @@
+<?php
+namespace GraphQL\Language\AST;
+
+class ObjectTypeDefinition extends Node implements TypeDefinition
+{
+    /**
+     * @var string
+     */
+    public $kind = Node::OBJECT_TYPE_DEFINITION;
+
+    /**
+     * @var Name
+     */
+    public $name;
+
+    /**
+     * @var NamedType[]
+     */
+    public $interfaces = [];
+
+    /**
+     * @var FieldDefinition[]
+     */
+    public $fields;
+}
\ No newline at end of file
diff --git a/src/Language/AST/OperationTypeDefinition.php b/src/Language/AST/OperationTypeDefinition.php
new file mode 100644
index 0000000..39be02d
--- /dev/null
+++ b/src/Language/AST/OperationTypeDefinition.php
@@ -0,0 +1,22 @@
+<?php
+namespace GraphQL\Language\AST;
+
+class OperationTypeDefinition extends Node
+{
+    /**
+     * @var string
+     */
+    public $kind = Node::OPERATION_TYPE_DEFINITION;
+
+    /**
+     * One of 'query' | 'mutation' | 'subscription'
+     *
+     * @var string
+     */
+    public $operation;
+
+    /**
+     * @var NamedType
+     */
+    public $type;
+}
diff --git a/src/Language/AST/ScalarTypeDefinition.php b/src/Language/AST/ScalarTypeDefinition.php
new file mode 100644
index 0000000..1a814f2
--- /dev/null
+++ b/src/Language/AST/ScalarTypeDefinition.php
@@ -0,0 +1,13 @@
+<?php
+namespace GraphQL\Language\AST;
+
+
+class ScalarTypeDefinition extends Node implements TypeDefinition
+{
+    public $kind = Node::SCALAR_TYPE_DEFINITION;
+
+    /**
+     * @var Name
+     */
+    public $name;
+}
diff --git a/src/Language/AST/SchemaDefinition.php b/src/Language/AST/SchemaDefinition.php
new file mode 100644
index 0000000..9f4440a
--- /dev/null
+++ b/src/Language/AST/SchemaDefinition.php
@@ -0,0 +1,15 @@
+<?php
+namespace GraphQL\Language\AST;
+
+class SchemaDefinition extends Node implements TypeSystemDefinition
+{
+    /**
+     * @var string
+     */
+    public $kind = Node::SCHEMA_DEFINITION;
+
+    /**
+     * @var OperationTypeDefinition[]
+     */
+    public $operationTypes;
+}
diff --git a/src/Language/AST/TypeDefinition.php b/src/Language/AST/TypeDefinition.php
new file mode 100644
index 0000000..503b64f
--- /dev/null
+++ b/src/Language/AST/TypeDefinition.php
@@ -0,0 +1,14 @@
+<?php
+namespace GraphQL\Language\AST;
+
+interface TypeDefinition extends TypeSystemDefinition
+{
+    /**
+    export type TypeDefinition = ScalarTypeDefinition
+    | ObjectTypeDefinition
+    | InterfaceTypeDefinition
+    | UnionTypeDefinition
+    | EnumTypeDefinition
+    | InputObjectTypeDefinition
+     */
+}
diff --git a/src/Language/AST/TypeExtensionDefinition.php b/src/Language/AST/TypeExtensionDefinition.php
new file mode 100644
index 0000000..7f24468
--- /dev/null
+++ b/src/Language/AST/TypeExtensionDefinition.php
@@ -0,0 +1,15 @@
+<?php
+namespace GraphQL\Language\AST;
+
+class TypeExtensionDefinition extends Node implements TypeSystemDefinition
+{
+    /**
+     * @var string
+     */
+    public $kind = Node::TYPE_EXTENSION_DEFINITION;
+
+    /**
+     * @var ObjectTypeDefinition
+     */
+    public $definition;
+}
diff --git a/src/Language/AST/TypeSystemDefinition.php b/src/Language/AST/TypeSystemDefinition.php
new file mode 100644
index 0000000..b5c4183
--- /dev/null
+++ b/src/Language/AST/TypeSystemDefinition.php
@@ -0,0 +1,12 @@
+<?php
+namespace GraphQL\Language\AST;
+
+interface TypeSystemDefinition extends Definition
+{
+    /**
+    export type TypeSystemDefinition = SchemaDefinition
+    | TypeDefinition
+    | TypeExtensionDefinition
+    | DirectiveDefinition
+     */
+}
diff --git a/src/Language/AST/UnionTypeDefinition.php b/src/Language/AST/UnionTypeDefinition.php
new file mode 100644
index 0000000..589c7ab
--- /dev/null
+++ b/src/Language/AST/UnionTypeDefinition.php
@@ -0,0 +1,20 @@
+<?php
+namespace GraphQL\Language\AST;
+
+class UnionTypeDefinition extends Node implements TypeDefinition
+{
+    /**
+     * @var string
+     */
+    public $kind = self::UNION_TYPE_DEFINITION;
+
+    /**
+     * @var Name
+     */
+    public $name;
+
+    /**
+     * @var NamedType[]
+     */
+    public $types = [];
+}
diff --git a/src/Language/Token.php b/src/Language/Token.php
index cdeac45..5bf7cd5 100644
--- a/src/Language/Token.php
+++ b/src/Language/Token.php
@@ -20,7 +20,6 @@ class Token
     const PIPE = 13;
     const BRACE_R = 14;
     const NAME = 15;
-    const VARIABLE = 16;
     const INT = 17;
     const FLOAT = 18;
     const STRING = 19;
@@ -43,7 +42,6 @@ class Token
         $description[self::PIPE] = '|';
         $description[self::BRACE_R] = '}';
         $description[self::NAME] = 'Name';
-        $description[self::VARIABLE] = 'Variable';
         $description[self::INT] = 'Int';
         $description[self::FLOAT] = 'Float';
         $description[self::STRING] = 'String';