diff --git a/src/Executor/Executor.php b/src/Executor/Executor.php
index 5418f64..1052841 100644
--- a/src/Executor/Executor.php
+++ b/src/Executor/Executor.php
@@ -19,6 +19,7 @@ use GraphQL\Type\Definition\ObjectType;
 use GraphQL\Type\Definition\ResolveInfo;
 use GraphQL\Type\Definition\ScalarType;
 use GraphQL\Type\Definition\Type;
+use GraphQL\Type\DefinitionContainer;
 use GraphQL\Type\Introspection;
 use GraphQL\Utils;
 
@@ -744,6 +745,10 @@ class Executor
             $runtimeType = $exeContext->schema->getType($runtimeType);
         }
 
+        if ($runtimeType instanceof DefinitionContainer) {
+            $runtimeType = $runtimeType->getDefinition();
+        }
+
         if (!($runtimeType instanceof ObjectType)) {
             throw new Error(
                 "Abstract type {$returnType} must resolve to an Object type at runtime " .
diff --git a/src/Schema.php b/src/Schema.php
index c9996ca..4002902 100644
--- a/src/Schema.php
+++ b/src/Schema.php
@@ -9,6 +9,7 @@ use GraphQL\Type\Definition\ObjectType;
 use GraphQL\Type\Definition\Type;
 use GraphQL\Type\Definition\UnionType;
 use GraphQL\Type\Definition\WrappingType;
+use GraphQL\Type\DefinitionContainer;
 use GraphQL\Type\Introspection;
 
 /**
@@ -113,6 +114,18 @@ class Schema
             'validate' => true
         ];
 
+        if ($config['query'] instanceof DefinitionContainer) {
+            $config['query'] = $config['query']->getDefinition();
+        }
+
+        if ($config['mutation'] instanceof DefinitionContainer) {
+            $config['mutation'] = $config['mutation']->getDefinition();
+        }
+
+        if ($config['subscription'] instanceof DefinitionContainer) {
+            $config['subscription'] = $config['subscription']->getDefinition();
+        }
+
         Utils::invariant(
             $config['query'] instanceof ObjectType,
             "Schema query must be Object Type but got: " . Utils::getVariableType($config['query'])
@@ -299,6 +312,9 @@ class Schema
         if (!$type) {
             return $this->typeMap;
         }
+        if ($type instanceof DefinitionContainer) {
+            $type = $type->getDefinition();
+        }
 
         if ($type instanceof WrappingType) {
             return $this->extractTypes($type->getWrappedType(true));
diff --git a/src/Type/Definition/Config.php b/src/Type/Definition/Config.php
index a561549..815b8c6 100644
--- a/src/Type/Definition/Config.php
+++ b/src/Type/Definition/Config.php
@@ -2,6 +2,7 @@
 namespace GraphQL\Type\Definition;
 
 use GraphQL\Error\InvariantViolation;
+use GraphQL\Type\DefinitionContainer;
 use GraphQL\Utils;
 
 /**
@@ -188,6 +189,10 @@ class Config
                 $err = 'Error in "'.$typeName.'" type definition: ' . "Each entry at '$pathStr' must be an array, but '%s' is '%s'";
 
                 foreach ($value as $arrKey => $arrValue) {
+                    if ($arrValue instanceof DefinitionContainer) {
+                        $arrValue = $arrValue->getDefinition();
+                    }
+
                     if (is_array($def->definition)) {
                         if ($def->flags & self::MAYBE_TYPE && $arrValue instanceof Type) {
                             $arrValue = ['type' => $arrValue];
@@ -220,6 +225,10 @@ class Config
                 return ; // Allow nulls for non-required fields
             }
 
+            if ($value instanceof DefinitionContainer) {
+                $value = $value->getDefinition();
+            }
+
             switch (true) {
                 case $def & self::ANY:
                     break;
diff --git a/src/Type/Definition/InterfaceType.php b/src/Type/Definition/InterfaceType.php
index a34da04..11f110a 100644
--- a/src/Type/Definition/InterfaceType.php
+++ b/src/Type/Definition/InterfaceType.php
@@ -52,7 +52,7 @@ class InterfaceType extends Type implements AbstractType, OutputType, CompositeT
     }
 
     /**
-     * @return array<FieldDefinition>
+     * @return FieldDefinition[]
      */
     public function getFields()
     {
diff --git a/src/Type/Definition/ListOfType.php b/src/Type/Definition/ListOfType.php
index b59ba74..3e37a54 100644
--- a/src/Type/Definition/ListOfType.php
+++ b/src/Type/Definition/ListOfType.php
@@ -1,6 +1,7 @@
 <?php
 namespace GraphQL\Type\Definition;
 
+use GraphQL\Type\DefinitionContainer;
 use GraphQL\Utils;
 
 /**
@@ -15,12 +16,12 @@ class ListOfType extends Type implements WrappingType, OutputType, InputType
     public $ofType;
 
     /**
-     * @param callable|Type $type
+     * @param callable|Type|DefinitionContainer $type
      */
     public function __construct($type)
     {
         Utils::invariant(
-            $type instanceof Type || is_callable($type),
+            $type instanceof Type || $type instanceof DefinitionContainer || is_callable($type),
             'Expecting instance of GraphQL\Type\Definition\Type or callable returning instance of that class'
         );
 
diff --git a/src/Type/Definition/NonNull.php b/src/Type/Definition/NonNull.php
index b4fe61f..cbdceb3 100644
--- a/src/Type/Definition/NonNull.php
+++ b/src/Type/Definition/NonNull.php
@@ -1,6 +1,7 @@
 <?php
 namespace GraphQL\Type\Definition;
 
+use GraphQL\Type\DefinitionContainer;
 use GraphQL\Type\TypeKind;
 use GraphQL\Utils;
 
@@ -16,13 +17,13 @@ class NonNull extends Type implements WrappingType, OutputType, InputType
     protected $ofType;
 
     /**
-     * @param callable|Type $type
+     * @param callable|Type|DefinitionContainer $type
      * @throws \Exception
      */
     public function __construct($type)
     {
         Utils::invariant(
-            $type instanceof Type || is_callable($type),
+            $type instanceof Type || $type instanceof DefinitionContainer || is_callable($type),
             'Expecting instance of GraphQL\Type\Definition\Type or callable returning instance of that class'
         );
         Utils::invariant(
diff --git a/src/Type/Definition/ObjectType.php b/src/Type/Definition/ObjectType.php
index e094a38..2c8c7f0 100644
--- a/src/Type/Definition/ObjectType.php
+++ b/src/Type/Definition/ObjectType.php
@@ -1,5 +1,6 @@
 <?php
 namespace GraphQL\Type\Definition;
+use GraphQL\Type\DefinitionContainer;
 use GraphQL\Utils;
 
 
@@ -139,6 +140,12 @@ class ObjectType extends Type implements OutputType, CompositeType
         if (null === $this->interfaces) {
             $interfaces = isset($this->config['interfaces']) ? $this->config['interfaces'] : [];
             $interfaces = is_callable($interfaces) ? call_user_func($interfaces) : $interfaces;
+
+            // TODO: Return some sort of generator to avoid multiple loops
+            $interfaces = Utils::map($interfaces, function($iface) {
+                return $iface instanceof DefinitionContainer ? $iface->getDefinition() : $iface;
+            });
+
             $this->interfaces = $interfaces;
         }
         return $this->interfaces;
diff --git a/src/Type/Definition/Type.php b/src/Type/Definition/Type.php
index f4a1467..cfe96e0 100644
--- a/src/Type/Definition/Type.php
+++ b/src/Type/Definition/Type.php
@@ -2,6 +2,7 @@
 namespace GraphQL\Type\Definition;
 
 use GraphQL\Error\InvariantViolation;
+use GraphQL\Type\DefinitionContainer;
 use GraphQL\Utils;
 
 /*
@@ -198,6 +199,9 @@ abstract class Type
             );
             $type = $type();
         }
+        if ($type instanceof DefinitionContainer) {
+            $type = $type->getDefinition();
+        }
 
         if (!$type instanceof Type) {
             throw new InvariantViolation(sprintf(
diff --git a/src/Type/Definition/UnionType.php b/src/Type/Definition/UnionType.php
index 876e3bb..90d26c0 100644
--- a/src/Type/Definition/UnionType.php
+++ b/src/Type/Definition/UnionType.php
@@ -1,6 +1,7 @@
 <?php
 namespace GraphQL\Type\Definition;
 
+use GraphQL\Type\DefinitionContainer;
 use GraphQL\Utils;
 
 /**
@@ -42,8 +43,6 @@ class UnionType extends Type implements AbstractType, OutputType, CompositeType
             'description' => Config::STRING
         ]);
 
-        Utils::invariant(!empty($config['types']), "");
-
         /**
          * Optionally provide a custom type resolver function. If one is not provided,
          * the default implemenation will call `isTypeOf` on each implementing
@@ -51,7 +50,6 @@ class UnionType extends Type implements AbstractType, OutputType, CompositeType
          */
         $this->name = $config['name'];
         $this->description = isset($config['description']) ? $config['description'] : null;
-        $this->types = $config['types'];
         $this->resolveTypeFn = isset($config['resolveType']) ? $config['resolveType'] : null;
         $this->config = $config;
     }
@@ -70,13 +68,23 @@ class UnionType extends Type implements AbstractType, OutputType, CompositeType
      */
     public function getTypes()
     {
-        if ($this->types instanceof \Closure) {
-            $this->types = call_user_func($this->types);
+        if (null === $this->types) {
+            if ($this->config['types'] instanceof \Closure) {
+                $types = call_user_func($this->config['types']);
+            } else {
+                $types = $this->config['types'];
+            }
+
             Utils::invariant(
-                is_array($this->types),
-                'Closure for option "types" of union "%s" is expected to return array of types',
+                is_array($types),
+                'Option "types" of union "%s" is expected to return array of types (or closure returning array of types)',
                 $this->name
             );
+
+            // TODO: Return some sort of generator to avoid multiple loops
+            $this->types = Utils::map($types, function($type) {
+                return $type instanceof DefinitionContainer ? $type->getDefinition() : $type;
+            });
         }
         return $this->types;
     }
diff --git a/src/Type/DefinitionContainer.php b/src/Type/DefinitionContainer.php
new file mode 100644
index 0000000..6cabc5e
--- /dev/null
+++ b/src/Type/DefinitionContainer.php
@@ -0,0 +1,15 @@
+<?php
+namespace GraphQL\Type;
+use GraphQL\Type\Definition\Type;
+
+/**
+ * Interface DefinitionContainer
+ * @package GraphQL\Type
+ */
+interface DefinitionContainer
+{
+    /**
+     * @return Type
+     */
+    public function getDefinition();
+}