diff --git a/docs/type-system/abstract-types.md b/docs/type-system/abstract-types.md
deleted file mode 100644
index 1c629d7..0000000
--- a/docs/type-system/abstract-types.md
+++ /dev/null
@@ -1,36 +0,0 @@
-# Interfaces
-In GraphQL an Interface is an abstract type that includes a certain set of fields that a 
-type must include to implement the interface.
-
-In **graphql-php** interface type is an instance of `GraphQL\Type\Definition\InterfaceType` 
-(or one of it subclasses) which accepts configuration array in constructor:
-
-```php
-use GraphQL\Type\Definition\InterfaceType;
-use GraphQL\Type\Definition\Type;
-
-$characterInterface = new InterfaceType([
-    'name' => 'Character',
-    'description' => 'A character in the Star Wars Trilogy',
-    'fields' => [
-        'id' => [
-            'type' => Type::nonNull(Type::string()),
-            'description' => 'The id of the character.',
-        ],
-        'name' => [
-            'type' => Type::string(),
-            'description' => 'The name of the character.'
-        ]
-    ],
-    'resolveType' => function ($obj) {
-        if ($obj->type === 'human') {
-            return MyTypes::human();            
-        } else {
-            return MyTypes::droid();
-        }
-        return null;
-    }
-]);
-```
-
-# Abstract Type Resolution
diff --git a/docs/type-system/interfaces.md b/docs/type-system/interfaces.md
new file mode 100644
index 0000000..9610ce0
--- /dev/null
+++ b/docs/type-system/interfaces.md
@@ -0,0 +1,127 @@
+# Interface Type Definition
+An Interface is an abstract type that includes a certain set of fields that a 
+type must include to implement the interface.
+
+In **graphql-php** interface type is an instance of `GraphQL\Type\Definition\InterfaceType` 
+(or one of it subclasses) which accepts configuration array in constructor:
+
+```php
+use GraphQL\Type\Definition\InterfaceType;
+use GraphQL\Type\Definition\Type;
+
+$character = new InterfaceType([
+    'name' => 'Character',
+    'description' => 'A character in the Star Wars Trilogy',
+    'fields' => [
+        'id' => [
+            'type' => Type::nonNull(Type::string()),
+            'description' => 'The id of the character.',
+        ],
+        'name' => [
+            'type' => Type::string(),
+            'description' => 'The name of the character.'
+        ]
+    ],
+    'resolveType' => function ($value) {
+        if ($value->type === 'human') {
+            return MyTypes::human();            
+        } else {
+            return MyTypes::droid();
+        }
+    }
+]);
+```
+This example uses **inline** style for Interface definition, but there are also 
+[other styles](/type-system/#type-definition-styles) (using inheritance or composition).
+
+# Configuration options
+Constructor of InterfaceType accepts an array. Below is a full list of allowed options:
+
+Option | Type | Notes
+------ | ---- | -----
+name | `string` | **Required.** Unique name of this interface type within Schema
+fields | `array` | **Required.** List of fields required to be defined by interface implementors. Same as [Fields for Object Type](#)
+description | `string` | Plain-text description of this type for clients (e.g. used by [GraphiQL](https://github.com/graphql/graphiql) for auto-generated documentation)
+resolveType | `callback` returning instance of `ObjectType` | **function($value, $context, GraphQL\Type\Definition\ResolveInfo $info)** Any `callable` that receives `$value` from resolver of the parent field and returns concrete interface implementor for that `$value`.
+
+# Implementing interface
+To implement the Interface simply add it to **interfaces** array of Object Type definition:
+```php
+$humanType = new ObjectType([
+    'name' => 'Human',
+    'fields' => [
+        'id' => [
+            'type' => Type::nonNull(Type::string()),
+            'description' => 'The id of the character.',
+        ],
+        'name' => [
+            'type' => Type::string(),
+            'description' => 'The name of the character.'
+        ]
+    ],
+    'interfaces' => [
+        $character
+    ]
+]);
+```
+Note that Object Type must include all fields of interface with exact same types 
+(including **nonNull** specification) and arguments.
+
+The only exception is when object's field type is more specific than the type of this field defined in interface 
+(see [Covariant return types for interface fields](#covariant-return-types-for-interface-fields) below)
+
+# Covariant return types for interface fields
+Object types implementing interface may change field type to more specific.
+Example:
+
+```
+interface A {
+  field1: A
+}
+
+type B implements A {
+  field1: B
+}
+```
+
+# Sharing Interface fields
+Since every Object Type implementing an Interface must have the same set of fields - it often makes 
+sense to re-use field definitions of Interface in Object Types:
+
+```php
+$humanType = new ObjectType([
+    'name' => 'Human',
+    'interfaces' => [
+        $character
+    ],
+    'fields' => [
+        'height' => Type::float(),
+        $character->getField('id'),
+        $character->getField('name')
+    ] 
+]);
+```
+
+In this case field definitions are created only once (as a part of Interface Type) and then 
+re-used by all interface implementors. It can save several microseconds and memory + ensures that 
+field definitions of Interface and implementors are always in sync.
+
+Yet it creates a problem with resolution of such fields. There are two ways how shared fields could 
+be resolved:
+
+1. If field resolution algorithm is the same for all Interface implementors - you can simply add 
+**resolve** option to field definition in Interface itself.
+
+2. If field resolution varies from implementor to implementor - you can specify **resolveField** 
+option in [Object Type config](/type-system/object-types/#configuration-options) and handle field 
+resolutions there 
+(Note: **resolve** option in field definition has precedence over **resolveField** option in object type definition)
+
+# Interface role in data fetching
+The only responsibility of interface in Data Fetching process is to return concrete Object Type 
+for given `$value` in **resolveType**. Then resolution of fields is delegated to resolvers of this 
+concrete Object Type.
+
+If **resolveType** option is omitted, **graphql-php** will loop through all interface implementors and 
+use their **isTypeOf** callback to pick the first suitable one. This is obviously less efficient 
+than single **resolveType** call. So it is recommended to define **resolveType** whenever possible.
diff --git a/docs/type-system/non-null.md b/docs/type-system/metadata.md
similarity index 100%
rename from docs/type-system/non-null.md
rename to docs/type-system/metadata.md
diff --git a/docs/type-system/unions.md b/docs/type-system/unions.md
new file mode 100644
index 0000000..a6cb7c7
--- /dev/null
+++ b/docs/type-system/unions.md
@@ -0,0 +1,36 @@
+# Union Type Definition
+A Union is an abstract type that simply enumerates other Object Types. 
+Value of Union Type is actually a value of one of included Object Types.
+
+In **graphql-php** union type is an instance of `GraphQL\Type\Definition\UnionType` 
+(or one of it subclasses) which accepts configuration array in constructor:
+
+```php
+$searchResultType = new UnionType([
+    'name' => 'SearchResult',
+    'types' => [
+        MyTypes::story(),
+        MyTypes::user()
+    ];
+    'resolveType' => function($value) {
+        if ($value->type === 'story') {
+            return MyTypes::story();            
+        } else {
+            return MyTypes::user();
+        }
+    }
+]);
+```
+
+This example uses **inline** style for Union definition, but there are also 
+[other styles](/type-system/#type-definition-styles) (using inheritance or composition).
+
+# Configuration options
+Constructor of UnionType accepts an array. Below is a full list of allowed options:
+
+Option | Type | Notes
+------ | ---- | -----
+name | `string` | **Required.** Unique name of this interface type within Schema
+types | `array` | **Required.** List of Object Types included in this Union. Note that you can't create a Union type out of Interfaces or other Unions.
+description | `string` | Plain-text description of this type for clients (e.g. used by [GraphiQL](https://github.com/graphql/graphiql) for auto-generated documentation)
+resolveType | `callback` returning instance of `ObjectType` | **function($value, $context, GraphQL\Type\Definition\ResolveInfo $info)** Any `callable` that receives `$value` from resolver of the parent field and returns Object Type for that `$value`.
diff --git a/mkdocs.yml b/mkdocs.yml
index 5ab30eb..f7b9691 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -8,8 +8,10 @@ pages:
   - Scalar Types: type-system/scalar-types.md
   - Enumeration Types: type-system/enum-types.md
   - Lists and Non-Null: type-system/lists-and-nonnulls.md
-  - Interfaces and Unions: type-system/abstract-types.md
+  - Interfaces: type-system/interfaces.md
+  - Unions: type-system/unions.md
   - Defining Schema: type-system/schema.md
+  - Custom Metadata: type-system/metadata.md
 - Data Fetching: data-fetching.md
 - Best Practices: best-practices.md
 - Complementary Tools: complementary-tools.md