diff --git a/.gitignore b/.gitignore
index 75af42eaa2..18ded80b82 100644
--- a/.gitignore
+++ b/.gitignore
@@ -18,6 +18,7 @@ gh-pages/
 tmp/*
 __*.db
 node_modules/
+.byebug_history
 yarn.lock
 OperationStoreClient.js
 spec/integration/tmp
diff --git a/lib/graphql/language/printer.rb b/lib/graphql/language/printer.rb
index e1dda9835c..e0d45d4470 100644
--- a/lib/graphql/language/printer.rb
+++ b/lib/graphql/language/printer.rb
@@ -161,6 +161,12 @@ def print_scalar_type_definition(scalar_type)
         out << print_directives(scalar_type.directives)
       end
 
+      def print_scalar_type_extension(scalar_type)
+        out = ''.dup
+        out << "extend scalar #{scalar_type.name}"
+        out << print_directives(scalar_type.directives)
+      end
+
       def print_object_type_definition(object_type)
         out = print_description(object_type)
         out << "type #{object_type.name}"
@@ -169,6 +175,14 @@ def print_object_type_definition(object_type)
         out << print_field_definitions(object_type.fields)
       end
 
+      def print_object_type_extension(object_type)
+        out = ''.dup
+        out << "extend type #{object_type.name}"
+        out << " implements " << object_type.interfaces.map(&:name).join(" & ") unless object_type.interfaces.empty?
+        out << print_directives(object_type.directives)
+        out << print_field_definitions(object_type.fields)
+      end
+
       def print_input_value_definition(input_value)
         out = "#{input_value.name}: #{print_node(input_value.type)}".dup
         out << " = #{print_node(input_value.default_value)}" unless input_value.default_value.nil?
@@ -204,6 +218,13 @@ def print_interface_type_definition(interface_type)
         out << print_field_definitions(interface_type.fields)
       end
 
+      def print_interface_type_extension(interface_type)
+        out = ''.dup
+        out << "extend interface #{interface_type.name}"
+        out << print_directives(interface_type.directives)
+        out << print_field_definitions(interface_type.fields)
+      end
+
       def print_union_type_definition(union_type)
         out = print_description(union_type)
         out << "union #{union_type.name}"
@@ -211,6 +232,13 @@ def print_union_type_definition(union_type)
         out << " = " + union_type.types.map(&:name).join(" | ")
       end
 
+      def print_union_type_extension(union_type)
+        out = ''.dup
+        out << "extend union #{union_type.name}"
+        out << print_directives(union_type.directives)
+        out << " = " + union_type.types.map(&:name).join(" | ")
+      end
+
       def print_enum_type_definition(enum_type)
         out = print_description(enum_type)
         out << "enum #{enum_type.name}#{print_directives(enum_type.directives)} {\n"
@@ -221,6 +249,16 @@ def print_enum_type_definition(enum_type)
         out << "}"
       end
 
+      def print_enum_type_extension(enum_type)
+        out = ''.dup
+        out << "extend enum #{enum_type.name}#{print_directives(enum_type.directives)} {\n"
+        enum_type.values.each.with_index do |value, i|
+          out << print_description(value, indent: '  ', first_in_block: i == 0)
+          out << print_enum_value_definition(value)
+        end
+        out << "}"
+      end
+
       def print_enum_value_definition(enum_value)
         out = "  #{enum_value.name}".dup
         out << print_directives(enum_value.directives)
@@ -239,6 +277,18 @@ def print_input_object_type_definition(input_object_type)
         out << "}"
       end
 
+      def print_input_object_type_extension(input_object_type)
+        out = ''.dup
+        out << "extend input #{input_object_type.name}"
+        out << print_directives(input_object_type.directives)
+        out << " {\n"
+        input_object_type.fields.each.with_index do |field, i|
+          out << print_description(field, indent: '  ', first_in_block: i == 0)
+          out << "  #{print_input_value_definition(field)}\n"
+        end
+        out << "}"
+      end
+
       def print_directive_definition(directive)
         out = print_description(directive)
         out << "directive @#{directive.name}"
@@ -342,6 +392,18 @@ def print_node(node, indent: "")
           print_input_object_type_definition(node)
         when Nodes::DirectiveDefinition
           print_directive_definition(node)
+        when Nodes::ScalarTypeExtension
+          print_scalar_type_extension(node)
+        when Nodes::ObjectTypeExtension
+          print_object_type_extension(node)
+        when Nodes::InterfaceTypeExtension
+          print_interface_type_extension(node)
+        when Nodes::UnionTypeExtension
+          print_union_type_extension(node)
+        when Nodes::EnumTypeExtension
+          print_enum_type_extension(node)
+        when Nodes::InputObjectTypeExtension
+          print_input_object_type_extension(node)
         when FalseClass, Float, Integer, NilClass, String, TrueClass, Symbol
           GraphQL::Language.serialize(node)
         when Array
diff --git a/lib/graphql/schema/build_from_definition.rb b/lib/graphql/schema/build_from_definition.rb
index 0e52c31c31..cd8e9b64f2 100644
--- a/lib/graphql/schema/build_from_definition.rb
+++ b/lib/graphql/schema/build_from_definition.rb
@@ -40,6 +40,7 @@ def build(document, default_resolve: DefaultResolve)
           types = {}
           types.merge!(GraphQL::Schema::BUILT_IN_TYPES)
           directives = {}
+          extensions = []
           type_resolver = ->(type) { -> { resolve_type(types, type) } }
 
           document.definitions.each do |definition|
@@ -47,6 +48,13 @@ def build(document, default_resolve: DefaultResolve)
             when GraphQL::Language::Nodes::SchemaDefinition
               raise InvalidDocumentError.new('Must provide only one schema definition.') if schema_definition
               schema_definition = definition
+            when GraphQL::Language::Nodes::ScalarTypeExtension,
+              GraphQL::Language::Nodes::ObjectTypeExtension,
+              GraphQL::Language::Nodes::InterfaceTypeExtension,
+              GraphQL::Language::Nodes::UnionTypeExtension,
+              GraphQL::Language::Nodes::EnumTypeExtension,
+              GraphQL::Language::Nodes::InputObjectTypeExtension
+              extensions << definition
             when GraphQL::Language::Nodes::EnumTypeDefinition
               types[definition.name] = build_enum_type(definition, type_resolver)
             when GraphQL::Language::Nodes::ObjectTypeDefinition
@@ -66,6 +74,32 @@ def build(document, default_resolve: DefaultResolve)
 
           directives = GraphQL::Schema.default_directives.merge(directives)
 
+          for ext in extensions
+            type = types[ext.name]
+            case ext
+            when GraphQL::Language::Nodes::ScalarTypeExtension
+              # TODO: Where are the directives on the scalar type?
+            when GraphQL::Language::Nodes::ObjectTypeExtension
+              type.fields.merge! Hash[build_fields(ext.fields, type_resolver, default_resolve: default_resolve)]
+              type.interfaces.concat ext.interfaces.map{ |interface_name| type_resolver.call(interface_name) }
+            when GraphQL::Language::Nodes::InterfaceTypeExtension
+              type.fields.merge! Hash[build_fields(ext.fields, type_resolver, default_resolve: nil)]
+            when GraphQL::Language::Nodes::UnionTypeExtension
+              type.possible_types.concat ext.types.map { |x|types[x.name] }
+            when GraphQL::Language::Nodes::EnumTypeExtension
+              ext.values.each do |enum_value_definition|
+                type.add_value EnumType::EnumValue.define(
+                  name: enum_value_definition.name,
+                  value: enum_value_definition.name,
+                  deprecation_reason: build_deprecation_reason(enum_value_definition.directives),
+                  description: enum_value_definition.description,
+                )
+              end
+            when GraphQL::Language::Nodes::InputObjectTypeExtension
+              type.arguments.merge! Hash[build_input_arguments(ext, type_resolver)]
+            end
+          end
+
           if schema_definition
             if schema_definition.query
               raise InvalidDocumentError.new("Specified query type \"#{schema_definition.query}\" not found in document.") unless types[schema_definition.query]
diff --git a/spec/graphql/language/extend_document_from_schema_definition_spec.rb b/spec/graphql/language/extend_document_from_schema_definition_spec.rb
new file mode 100644
index 0000000000..6d1b169823
--- /dev/null
+++ b/spec/graphql/language/extend_document_from_schema_definition_spec.rb
@@ -0,0 +1,104 @@
+# frozen_string_literal: true
+require "spec_helper"
+
+describe GraphQL::Language::DocumentFromSchemaDefinition do
+  let(:subject) { GraphQL::Language::DocumentFromSchemaDefinition }
+
+  describe "#document" do
+    let(:schema_idl) { <<-GRAPHQL
+      scalar SomeScalar
+      extend scalar SomeScalar @foo
+
+      type Foo {
+        name: String
+      }
+
+      type User {
+        name: String
+      }
+
+      type Bar {
+        name: String
+      }
+
+      union SomeUnion = User | Foo
+      extend union SomeUnion = Bar
+
+      extend type User {
+        email: String!
+      }
+
+      interface SomeInterface {
+        otherField: String
+      }
+
+      extend interface SomeInterface {
+        newField: String
+      }
+
+      input SomeInput {
+        fooArg: String
+        newField: String
+      }
+
+      extend input SomeInput {
+        newField: String
+      }
+
+      enum SomeEnum {
+        ONE
+      }
+
+      extend enum SomeEnum {
+        NEW_ENUM
+      }
+
+      type Query {
+        user: User
+      }
+
+      GRAPHQL
+    }
+
+    let(:expected_document) { GraphQL.parse(schema_idl) }
+    let(:schema) { GraphQL::Schema.from_definition(schema_idl) }
+
+    let(:document) {
+      subject.new(
+        schema
+      ).document
+    }
+
+    it "returns the IDL without introspection, built ins and schema root" do
+      assert equivalent_node?(expected_document, document)
+    end
+
+    it "print GraphQL::Language::Document with extend" do
+      document = GraphQL.parse(schema_idl)
+      GraphQL::Language::Printer.new.print(document)
+    end
+
+    it "generates GraphQL::Language::Document from the GraphQL::Schema build" do
+    end
+  end
+
+  private
+
+  def equivalent_node?(expected, node)
+    return false unless expected.is_a?(node.class)
+
+    if expected.respond_to?(:children) && expected.respond_to?(:scalars)
+      children_equal = expected.children.all? do |expected_child|
+        node.children.find { |child| equivalent_node?(expected_child, child) }
+      end
+
+      scalars_equal = expected.children.all? do |expected_child|
+        node.children.find { |child| equivalent_node?(expected_child, child) }
+      end
+
+      children_equal && scalars_equal
+    else
+      expected == node
+    end
+  end
+end