diff --git a/src/Language/GraphQL/AST.hs b/src/Language/GraphQL/AST.hs
index 06aeedf..ea7912a 100644
--- a/src/Language/GraphQL/AST.hs
+++ b/src/Language/GraphQL/AST.hs
@@ -1,7 +1,4 @@
--- | This module defines an abstract syntax tree for the @GraphQL@ language based on
---   <https://facebook.github.io/graphql/ Facebook's GraphQL Specification>.
---
--- Target AST for Parser.
+-- | Target AST for Parser.
 module Language.GraphQL.AST
     ( Alias
     , Argument(..)
diff --git a/src/Language/GraphQL/AST/DirectiveLocation.hs b/src/Language/GraphQL/AST/DirectiveLocation.hs
index 12c7e18..5b7a36f 100644
--- a/src/Language/GraphQL/AST/DirectiveLocation.hs
+++ b/src/Language/GraphQL/AST/DirectiveLocation.hs
@@ -1,14 +1,20 @@
+-- | Various parts of a GraphQL document can be annotated with directives. 
+--   This module describes locations in a document where directives can appear.
 module Language.GraphQL.AST.DirectiveLocation
-    ( DirectiveLocation
-    , ExecutableDirectiveLocation
-    , TypeSystemDirectiveLocation
+    ( DirectiveLocation(..)
+    , ExecutableDirectiveLocation(..)
+    , TypeSystemDirectiveLocation(..)
     ) where
 
+-- | All directives can be splitted in two groups: directives used to annotate
+--   various parts of executable definitions and the ones used in the schema
+--   definition.
 data DirectiveLocation
     = ExecutableDirectiveLocation ExecutableDirectiveLocation
     | TypeSystemDirectiveLocation TypeSystemDirectiveLocation
     deriving (Eq, Show)
 
+-- | Where directives can appear in an executable definition, like a query.
 data ExecutableDirectiveLocation
     = Query
     | Mutation
@@ -19,6 +25,7 @@ data ExecutableDirectiveLocation
     | InlineFragment
     deriving (Eq, Show)
 
+-- | Where directives can appear in a type system definition.
 data TypeSystemDirectiveLocation
     = Schema
     | Scalar
diff --git a/src/Language/GraphQL/AST/Document.hs b/src/Language/GraphQL/AST/Document.hs
index 0350ce1..ebe8918 100644
--- a/src/Language/GraphQL/AST/Document.hs
+++ b/src/Language/GraphQL/AST/Document.hs
@@ -1,4 +1,7 @@
--- | This module defines data structures representing a GraphQL document.
+-- | This module defines an abstract syntax tree for the @GraphQL@ language. It
+--   follows closely the structure given in the specification. Please refer to
+--   <https://facebook.github.io/graphql/ Facebook's GraphQL Specification>.
+--   for more information.
 module Language.GraphQL.AST.Document
     ( Definition(..)
     , Document
@@ -17,20 +20,15 @@ import Language.GraphQL.AST
     )
 import Language.GraphQL.AST.DirectiveLocation
 
+-- * Language
+
+-- ** Document
+
 -- | GraphQL document.
 type Document = NonEmpty Definition
 
 type NamedType = Name
 
-newtype Description = Description (Maybe Text)
-    deriving (Eq, Show)
-
-type RootOperationTypeDefinitions = NonEmpty RootOperationTypeDefinition
-
-data RootOperationTypeDefinition
-    = RootOperationTypeDefinition OperationType NamedType
-    deriving (Eq, Show)
-
 -- | All kinds of definitions that can occur in a GraphQL document.
 data Definition
     = ExecutableDefinition ExecutableDefinition
@@ -38,70 +36,108 @@ data Definition
     | TypeSystemExtension TypeSystemExtension
     deriving (Eq, Show)
 
+-- * Type System
+
 data TypeSystemDefinition
     = SchemaDefinition [Directive] RootOperationTypeDefinitions
     | TypeDefinition TypeDefinition
     | DirectiveDefinition Description Name ArgumentsDefinition DirectiveLocation
     deriving (Eq, Show)
 
+-- ** Type System Extensions
+
+data TypeSystemExtension
+    = SchemaExtension SchemaExtension
+    | TypeExtension TypeExtension
+    deriving (Eq, Show)
+
+-- ** Schema
+
+type RootOperationTypeDefinitions = NonEmpty RootOperationTypeDefinition
+
+data RootOperationTypeDefinition
+    = RootOperationTypeDefinition OperationType NamedType
+    deriving (Eq, Show)
+
 data SchemaExtension
     = SchemaOperationExtension [Directive] RootOperationTypeDefinitions
     | SchemaDirectiveExtension (NonEmpty Directive)
     deriving (Eq, Show)
 
-data TypeSystemExtension
-    = SchemaExtension SchemaExtension
-    | TypeExtension TypeExtension
+-- ** Descriptions
+
+newtype Description = Description (Maybe Text)
     deriving (Eq, Show)
 
+-- ** Types
+
+data TypeDefinition
+    = ScalarTypeDefinition Description Name [Directive]
+    | ObjectTypeDefinition
+        Description Name ImplementsInterfacesOpt [Directive] [FieldDefinition]
+    | InterfaceTypeDefinition Description Name [Directive] [FieldDefinition]
+    | UnionTypeDefinition Description Name [Directive] UnionMemberTypesOpt
+    | EnumTypeDefinition Description Name [Directive] [EnumValueDefinition]
+    | InputObjectTypeDefinition
+        Description Name [Directive] InputFieldsDefinitionOpt
+    deriving (Eq, Show)
+
+data TypeExtension
+    = ScalarTypeExtension Name (NonEmpty Directive)
+    | ObjectTypeFieldsDefinitionExtension
+        Name ImplementsInterfacesOpt [Directive] (NonEmpty FieldDefinition)
+    | ObjectTypeDirectivesExtension
+        Name ImplementsInterfacesOpt (NonEmpty Directive)
+    | ObjectTypeImplementsInterfacesExtension Name ImplementsInterfaces
+    | InterfaceTypeFieldsDefinitionExtension
+        Name [Directive] (NonEmpty FieldDefinition)
+    | InterfaceTypeDirectivesExtension Name (NonEmpty Directive)
+    | UnionTypeUnionMemberTypesExtension Name [Directive] UnionMemberTypes
+    | UnionDirectivesExtension Name (NonEmpty Directive)
+    | EnumTypeEnumValuesDefinitionExtension
+        Name [Directive] (NonEmpty EnumValueDefinition)
+    | EnumTypeDirectivesExtension Name (NonEmpty Directive)
+    | InputObjectTypeInputFieldsDefinitionExtension
+        Name [Directive] InputFieldsDefinition
+    | InputObjectTypeDirectivesExtension Name (NonEmpty Directive)
+    deriving (Eq, Show)
+
+-- ** Objects
+
 newtype ImplementsInterfaces = ImplementsInterfaces (NonEmpty NamedType)
     deriving (Eq, Show)
 newtype ImplementsInterfacesOpt = ImplementsInterfacesOpt [NamedType]
     deriving (Eq, Show)
 
-newtype UnionMemberTypes = UnionMemberTypes (NonEmpty NamedType)
-    deriving (Eq, Show)
-newtype UnionMemberTypesOpt = UnionMemberTypesOpt [NamedType]
+data FieldDefinition = FieldDefinition Description Name ArgumentsDefinition Type
     deriving (Eq, Show)
 
-newtype InputFieldsDefinition = InputFieldsDefinition (NonEmpty InputValueDefinition)
-    deriving (Eq, Show)
-newtype InputFieldsDefinitionOpt = InputFieldsDefinitionOpt [InputValueDefinition]
+newtype ArgumentsDefinition = ArgumentsDefinition [InputValueDefinition]
     deriving (Eq, Show)
 
 data InputValueDefinition
     = InputValueDefinition Description Name Type (Maybe Value) [Directive]
     deriving (Eq, Show)
 
-newtype ArgumentsDefinition = ArgumentsDefinition [InputValueDefinition]
+-- ** Unions
+
+newtype UnionMemberTypes = UnionMemberTypes (NonEmpty NamedType)
     deriving (Eq, Show)
 
+newtype UnionMemberTypesOpt = UnionMemberTypesOpt [NamedType]
+    deriving (Eq, Show)
+
+-- ** Enums
+
 data EnumValueDefinition = EnumValueDefinition Description Name [Directive]
     deriving (Eq, Show)
 
-data FieldDefinition = FieldDefinition Description Name ArgumentsDefinition Type
+-- ** Input Objects
+
+newtype InputFieldsDefinition
+    = InputFieldsDefinition (NonEmpty InputValueDefinition)
     deriving (Eq, Show)
 
-data TypeDefinition
-    = ScalarTypeDefinition Description Name [Directive]
-    | ObjectTypeDefinition Description Name ImplementsInterfacesOpt [Directive] [FieldDefinition]
-    | InterfaceTypeDefinition Description Name [Directive] [FieldDefinition]
-    | UnionTypeDefinition Description Name [Directive] UnionMemberTypesOpt
-    | EnumTypeDefinition Description Name [Directive] [EnumValueDefinition]
-    | InputObjectTypeDefinition Description Name [Directive] InputFieldsDefinitionOpt
-    deriving (Eq, Show)
-
-data TypeExtension
-    = ScalarTypeExtension Name (NonEmpty Directive)
-    | ObjectTypeFieldsDefinitionExtension Name ImplementsInterfacesOpt [Directive] (NonEmpty FieldDefinition)
-    | ObjectTypeDirectivesExtension Name ImplementsInterfacesOpt (NonEmpty Directive)
-    | ObjectTypeImplementsInterfacesExtension Name ImplementsInterfaces
-    | InterfaceTypeFieldsDefinitionExtension Name [Directive] (NonEmpty FieldDefinition)
-    | InterfaceTypeDirectivesExtension Name (NonEmpty Directive)
-    | UnionTypeUnionMemberTypesExtension Name [Directive] UnionMemberTypes
-    | UnionDirectivesExtension Name (NonEmpty Directive)
-    | EnumTypeEnumValuesDefinitionExtension Name [Directive] (NonEmpty EnumValueDefinition)
-    | EnumTypeDirectivesExtension Name (NonEmpty Directive)
-    | InputObjectTypeInputFieldsDefinitionExtension Name [Directive] InputFieldsDefinition
-    | InputObjectTypeDirectivesExtension Name (NonEmpty Directive)
+newtype InputFieldsDefinitionOpt
+    = InputFieldsDefinitionOpt [InputValueDefinition]
     deriving (Eq, Show)