diff --git a/src/Executor/Executor.php b/src/Executor/Executor.php
index 0530a60..011e7cf 100644
--- a/src/Executor/Executor.php
+++ b/src/Executor/Executor.php
@@ -969,9 +969,59 @@ class Executor
         $runtimeType = $returnType->resolveType($result, $exeContext->contextValue, $info);
 
         if (null === $runtimeType) {
-            $runtimeType = self::inferTypeOf($result, $exeContext->contextValue, $info, $returnType);
+            $runtimeType = self::defaultTypeResolver($result, $exeContext->contextValue, $info, $returnType);
         }
 
+        if ($this->promises->isThenable($runtimeType)) {
+            $runtimeType = $this->promises->convertThenable($runtimeType);
+            Utils::invariant($runtimeType instanceof Promise);
+        }
+
+        if ($runtimeType instanceof Promise) {
+            return $runtimeType->then(function($resolvedRuntimeType) use ($returnType, $fieldNodes, $info, $path, &$result) {
+                return $this->validateRuntimeTypeAndCompleteObjectValue(
+                    $returnType,
+                    $fieldNodes,
+                    $info,
+                    $path,
+                    $resolvedRuntimeType,
+                    $result
+                );
+            });
+        }
+
+        return $this->validateRuntimeTypeAndCompleteObjectValue(
+            $returnType,
+            $fieldNodes,
+            $info,
+            $path,
+            $runtimeType,
+            $result
+        );
+    }
+
+    /**
+     * @param AbstractType $returnType
+     * @param FieldNode[] $fieldNodes
+     * @param ResolveInfo $info
+     * @param array $path
+     * @param mixed $returnedRuntimeType
+     * @param $result
+     * @return array|Promise|\stdClass
+     * @throws Error
+     */
+    private function validateRuntimeTypeAndCompleteObjectValue(
+        AbstractType $returnType,
+        $fieldNodes,
+        ResolveInfo $info,
+        $path,
+        $returnedRuntimeType,
+        &$result
+    )
+    {
+        $exeContext = $this->exeContext;
+        $runtimeType = $returnedRuntimeType;
+
         // If resolveType returns a string, we assume it's a ObjectType name.
         if (is_string($runtimeType)) {
             $runtimeType = $exeContext->schema->getType($runtimeType);
@@ -1064,12 +1114,61 @@ class Executor
      */
     private function completeObjectValue(ObjectType $returnType, $fieldNodes, ResolveInfo $info, $path, &$result)
     {
-        $exeContext = $this->exeContext;
+        $isTypeOf = $returnType->isTypeOf($result, $this->exeContext->contextValue, $info);
 
-        // If there is an isTypeOf predicate function, call it with the
-        // current result. If isTypeOf returns false, then raise an error rather
-        // than continuing execution.
-        if (false === $returnType->isTypeOf($result, $exeContext->contextValue, $info)) {
+        if (null === $isTypeOf) {
+            return $this->validateResultTypeAndExecuteFields(
+                $returnType,
+                $fieldNodes,
+                $info,
+                $path,
+                $result,
+                true
+            );
+        }
+
+        if ($this->promises->isThenable($isTypeOf)) {
+            /** @var Promise $isTypeOf */
+            $isTypeOf = $this->promises->convertThenable($isTypeOf);
+            Utils::invariant($isTypeOf instanceof Promise);
+
+            return $isTypeOf->then(function($isTypeOfResult) use ($returnType, $fieldNodes, $info, $path, &$result) {
+                return $this->validateResultTypeAndExecuteFields(
+                    $returnType,
+                    $fieldNodes,
+                    $info,
+                    $path,
+                    $result,
+                    $isTypeOfResult
+                );
+            });
+        }
+
+        return $this->validateResultTypeAndExecuteFields($returnType, $fieldNodes, $info, $path, $result, $isTypeOf);
+    }
+
+    /**
+     * @param ObjectType $returnType
+     * @param FieldNode[] $fieldNodes
+     * @param ResolveInfo $info
+     * @param array $path
+     * @param array $result
+     * @param bool $isTypeOfResult
+     * @return array|Promise|\stdClass
+     * @throws Error
+     */
+    private function validateResultTypeAndExecuteFields(
+        ObjectType $returnType,
+        $fieldNodes,
+        ResolveInfo $info,
+        $path,
+        &$result,
+        $isTypeOfResult
+    )
+    {
+        // If isTypeOf returns false, then raise an error
+        // rather than continuing execution.
+        if (false === $isTypeOfResult) {
             throw new Error(
                 "Expected value of type $returnType but got: " . Utils::getVariableType($result),
                 $fieldNodes
@@ -1095,23 +1194,47 @@ class Executor
     }
 
     /**
-     * Infer type of the value using isTypeOf of corresponding AbstractType
+     * If a resolveType function is not given, then a default resolve behavior is
+     * used which tests each possible type for the abstract type by calling
+     * isTypeOf for the object being coerced, returning the first type that matches.
      *
      * @param $value
      * @param $context
      * @param ResolveInfo $info
      * @param AbstractType $abstractType
-     * @return ObjectType|null
+     * @return ObjectType|Promise|null
      */
-    private static function inferTypeOf($value, $context, ResolveInfo $info, AbstractType $abstractType)
+    private function defaultTypeResolver($value, $context, ResolveInfo $info, AbstractType $abstractType)
     {
         $possibleTypes = $info->schema->getPossibleTypes($abstractType);
+        $promisedIsTypeOfResults = [];
+        $promisedIsTypeOfResultTypes = [];
 
         foreach ($possibleTypes as $type) {
-            if ($type->isTypeOf($value, $context, $info)) {
-                return $type;
+            $isTypeOfResult = $type->isTypeOf($value, $context, $info);
+
+            if (null !== $isTypeOfResult) {
+                if ($this->promises->isThenable($isTypeOfResult)) {
+                    $promisedIsTypeOfResults[] = $this->promises->convertThenable($isTypeOfResult);
+                    $promisedIsTypeOfResultTypes[] = $type;
+                } else if ($isTypeOfResult) {
+                    return $type;
+                }
             }
         }
+
+        if (!empty($promisedIsTypeOfResults)) {
+            return $this->promises->all($promisedIsTypeOfResults)
+                ->then(function($isTypeOfResults) use ($promisedIsTypeOfResultTypes) {
+                    foreach ($isTypeOfResults as $index => $result) {
+                        if ($result) {
+                            return $promisedIsTypeOfResultTypes[$index];
+                        }
+                    }
+                    return null;
+                });
+        }
+
         return null;
     }
 
diff --git a/tests/Executor/AbstractPromiseTest.php b/tests/Executor/AbstractPromiseTest.php
new file mode 100644
index 0000000..1a6894a
--- /dev/null
+++ b/tests/Executor/AbstractPromiseTest.php
@@ -0,0 +1,654 @@
+<?php
+namespace GraphQL\Tests\Executor;
+
+use GraphQL\Deferred;
+use GraphQL\GraphQL;
+use GraphQL\Schema;
+use GraphQL\Type\Definition\InterfaceType;
+use GraphQL\Type\Definition\ObjectType;
+use GraphQL\Type\Definition\Type;
+use GraphQL\Type\Definition\UnionType;
+
+require_once __DIR__ . '/TestClasses.php';
+
+class AbstractPromiseTest extends \PHPUnit_Framework_TestCase
+{
+    // DESCRIBE: Execute: Handles execution of abstract types with promises
+
+    /**
+     * @it isTypeOf used to resolve runtime type for Interface
+     */
+    public function testIsTypeOfUsedToResolveRuntimeTypeForInterface()
+    {
+        $PetType = new InterfaceType([
+            'name' => 'Pet',
+            'fields' => [
+                'name' => [ 'type' => Type::string() ]
+            ]
+        ]);
+
+        $DogType = new ObjectType([
+            'name' => 'Dog',
+            'interfaces' => [ $PetType ],
+            'isTypeOf' => function($obj) {
+                return new Deferred(function() use ($obj) {
+                    return $obj instanceof Dog;
+                });
+            },
+            'fields' => [
+                'name' => [ 'type' => Type::string() ],
+                'woofs' => [ 'type' => Type::boolean() ],
+            ]
+        ]);
+
+        $CatType = new ObjectType([
+            'name' => 'Cat',
+            'interfaces' => [ $PetType ],
+            'isTypeOf' => function($obj) {
+                return new Deferred(function() use ($obj) {
+                    return $obj instanceof Cat;
+                });
+            },
+            'fields' => [
+                'name' => [ 'type' => Type::string() ],
+                'meows' => [ 'type' => Type::boolean() ],
+            ]
+        ]);
+
+        $schema = new Schema([
+            'query' => new ObjectType([
+                'name' => 'Query',
+                'fields' => [
+                    'pets' => [
+                        'type' => Type::listOf($PetType),
+                        'resolve' => function() {
+                            return [
+                                new Dog('Odie', true),
+                                new Cat('Garfield', false)
+                            ];
+                        }
+                    ]
+                ]
+            ]),
+            'types' => [ $CatType, $DogType ]
+        ]);
+
+        $query = '{
+          pets {
+            name
+            ... on Dog {
+              woofs
+            }
+            ... on Cat {
+              meows
+            }
+          }
+        }';
+
+        $result = GraphQL::execute($schema, $query);
+
+        $expected = [
+            'data' => [
+                'pets' => [
+                    [ 'name' => 'Odie', 'woofs' => true ],
+                    [ 'name' => 'Garfield', 'meows' => false ]
+                ]
+            ]
+        ];
+
+        $this->assertEquals($expected, $result);
+    }
+
+    /**
+     * @it isTypeOf can be rejected
+     */
+    public function testIsTypeOfCanBeRejected()
+    {
+
+        $PetType = new InterfaceType([
+            'name' => 'Pet',
+            'fields' => [
+                'name' => ['type' => Type::string()]
+            ]
+        ]);
+
+        $DogType = new ObjectType([
+            'name' => 'Dog',
+            'interfaces' => [$PetType],
+            'isTypeOf' => function () {
+                return new Deferred(function () {
+                    throw new \Exception('We are testing this error');
+                });
+            },
+            'fields' => [
+                'name' => ['type' => Type::string()],
+                'woofs' => ['type' => Type::boolean()],
+            ]
+        ]);
+
+        $CatType = new ObjectType([
+            'name' => 'Cat',
+            'interfaces' => [$PetType],
+            'isTypeOf' => function ($obj) {
+                return new Deferred(function () use ($obj) {
+                    return $obj instanceof Cat;
+                });
+            },
+            'fields' => [
+                'name' => ['type' => Type::string()],
+                'meows' => ['type' => Type::boolean()],
+            ]
+        ]);
+
+        $schema = new Schema([
+            'query' => new ObjectType([
+                'name' => 'Query',
+                'fields' => [
+                    'pets' => [
+                        'type' => Type::listOf($PetType),
+                        'resolve' => function () {
+                            return [
+                                new Dog('Odie', true),
+                                new Cat('Garfield', false)
+                            ];
+                        }
+                    ]
+                ]
+            ]),
+            'types' => [$CatType, $DogType]
+        ]);
+
+        $query = '{
+      pets {
+        name
+        ... on Dog {
+          woofs
+        }
+        ... on Cat {
+          meows
+        }
+      }
+    }';
+
+        $result = GraphQL::execute($schema, $query);
+
+        $expected = [
+            'data' => [
+                'pets' => [null, null]
+            ],
+            'errors' => [
+                [
+                    'message' => 'We are testing this error',
+                    'locations' => [['line' => 2, 'column' => 7]],
+                    'path' => ['pets', 0]
+                ],
+                [
+                    'message' => 'We are testing this error',
+                    'locations' => [['line' => 2, 'column' => 7]],
+                    'path' => ['pets', 1]
+                ]
+            ]
+        ];
+
+        $this->assertEquals($expected, $result);
+    }
+
+    /**
+     * @it isTypeOf used to resolve runtime type for Union
+     */
+    public function testIsTypeOfUsedToResolveRuntimeTypeForUnion()
+    {
+
+        $DogType = new ObjectType([
+            'name' => 'Dog',
+            'isTypeOf' => function ($obj) {
+                return new Deferred(function () use ($obj) {
+                    return $obj instanceof Dog;
+                });
+            },
+            'fields' => [
+                'name' => ['type' => Type::string()],
+                'woofs' => ['type' => Type::boolean()],
+            ]
+        ]);
+
+        $CatType = new ObjectType([
+            'name' => 'Cat',
+            'isTypeOf' => function ($obj) {
+                return new Deferred(function () use ($obj) {
+                    return $obj instanceof Cat;
+                });
+            },
+            'fields' => [
+                'name' => ['type' => Type::string()],
+                'meows' => ['type' => Type::boolean()],
+            ]
+        ]);
+
+        $PetType = new UnionType([
+            'name' => 'Pet',
+            'types' => [$DogType, $CatType]
+        ]);
+
+        $schema = new Schema([
+            'query' => new ObjectType([
+                'name' => 'Query',
+                'fields' => [
+                    'pets' => [
+                        'type' => Type::listOf($PetType),
+                        'resolve' => function () {
+                            return [new Dog('Odie', true), new Cat('Garfield', false)];
+                        }
+                    ]
+                ]
+            ])
+        ]);
+
+        $query = '{
+      pets {
+        ... on Dog {
+          name
+          woofs
+        }
+        ... on Cat {
+          name
+          meows
+        }
+      }
+    }';
+
+        $result = GraphQL::execute($schema, $query);
+
+        $expected = [
+            'data' => [
+                'pets' => [
+                    ['name' => 'Odie', 'woofs' => true],
+                    ['name' => 'Garfield', 'meows' => false]
+                ]
+            ]
+        ];
+
+        $this->assertEquals($expected, $result);
+    }
+
+    /**
+     * @it resolveType on Interface yields useful error
+     */
+    public function testResolveTypeOnInterfaceYieldsUsefulError()
+    {
+        $PetType = new InterfaceType([
+            'name' => 'Pet',
+            'resolveType' => function ($obj) use (&$DogType, &$CatType, &$HumanType) {
+                return new Deferred(function () use ($obj, $DogType, $CatType, $HumanType) {
+                    if ($obj instanceof Dog) {
+                        return $DogType;
+                    }
+                    if ($obj instanceof Cat) {
+                        return $CatType;
+                    }
+                    if ($obj instanceof Human) {
+                        return $HumanType;
+                    }
+                    return null;
+                });
+            },
+            'fields' => [
+                'name' => ['type' => Type::string()]
+            ]
+        ]);
+
+        $HumanType = new ObjectType([
+            'name' => 'Human',
+            'fields' => [
+                'name' => ['type' => Type::string()],
+            ]
+        ]);
+
+        $DogType = new ObjectType([
+            'name' => 'Dog',
+            'interfaces' => [$PetType],
+            'fields' => [
+                'name' => ['type' => Type::string()],
+                'woofs' => ['type' => Type::boolean()],
+            ]
+        ]);
+
+        $CatType = new ObjectType([
+            'name' => 'Cat',
+            'interfaces' => [$PetType],
+            'fields' => [
+                'name' => ['type' => Type::string()],
+                'meows' => ['type' => Type::boolean()],
+            ]
+        ]);
+
+        $schema = new Schema([
+            'query' => new ObjectType([
+                'name' => 'Query',
+                'fields' => [
+                    'pets' => [
+                        'type' => Type::listOf($PetType),
+                        'resolve' => function () {
+                            return new Deferred(function () {
+                                return [
+                                    new Dog('Odie', true),
+                                    new Cat('Garfield', false),
+                                    new Human('Jon')
+                                ];
+                            });
+                        }
+                    ]
+                ]
+            ]),
+            'types' => [$CatType, $DogType]
+        ]);
+
+        $query = '{
+      pets {
+        name
+        ... on Dog {
+          woofs
+        }
+        ... on Cat {
+          meows
+        }
+      }
+    }';
+
+        $result = GraphQL::execute($schema, $query);
+
+        $expected = [
+            'data' => [
+                'pets' => [
+                    ['name' => 'Odie', 'woofs' => true],
+                    ['name' => 'Garfield', 'meows' => false],
+                    null
+                ]
+            ],
+            'errors' => [
+                [
+                    'message' => 'Runtime Object type "Human" is not a possible type for "Pet".',
+                    'locations' => [['line' => 2, 'column' => 7]],
+                    'path' => ['pets', 2]
+                ],
+            ]
+        ];
+
+        $this->assertEquals($expected, $result);
+    }
+
+    /**
+     * @it resolveType on Union yields useful error
+     */
+    public function testResolveTypeOnUnionYieldsUsefulError()
+    {
+
+        $HumanType = new ObjectType([
+            'name' => 'Human',
+            'fields' => [
+                'name' => ['type' => Type::string()],
+            ]
+        ]);
+
+        $DogType = new ObjectType([
+            'name' => 'Dog',
+            'fields' => [
+                'name' => ['type' => Type::string()],
+                'woofs' => ['type' => Type::boolean()],
+            ]
+        ]);
+
+        $CatType = new ObjectType([
+            'name' => 'Cat',
+            'fields' => [
+                'name' => ['type' => Type::string()],
+                'meows' => ['type' => Type::boolean()],
+            ]
+        ]);
+
+        $PetType = new UnionType([
+            'name' => 'Pet',
+            'resolveType' => function ($obj) use ($DogType, $CatType, $HumanType) {
+                return new Deferred(function () use ($obj, $DogType, $CatType, $HumanType) {
+                    if ($obj instanceof Dog) {
+                        return $DogType;
+                    }
+                    if ($obj instanceof Cat) {
+                        return $CatType;
+                    }
+                    if ($obj instanceof Human) {
+                        return $HumanType;
+                    }
+                    return null;
+                });
+            },
+            'types' => [$DogType, $CatType]
+        ]);
+
+        $schema = new Schema([
+            'query' => new ObjectType([
+                'name' => 'Query',
+                'fields' => [
+                    'pets' => [
+                        'type' => Type::listOf($PetType),
+                        'resolve' => function () {
+                            return [
+                                new Dog('Odie', true),
+                                new Cat('Garfield', false),
+                                new Human('Jon')
+                            ];
+                        }
+                    ]
+                ]
+            ])
+        ]);
+
+        $query = '{
+      pets {
+        ... on Dog {
+          name
+          woofs
+        }
+        ... on Cat {
+          name
+          meows
+        }
+      }
+    }';
+
+        $result = GraphQL::execute($schema, $query);
+
+        $expected = [
+            'data' => [
+                'pets' => [
+                    ['name' => 'Odie', 'woofs' => true],
+                    ['name' => 'Garfield', 'meows' => false],
+                    null
+                ]
+            ],
+            'errors' => [
+                [
+                    'message' => 'Runtime Object type "Human" is not a possible type for "Pet".',
+                    'locations' => [['line' => 2, 'column' => 7]],
+                    'path' => ['pets', 2]
+                ]
+            ]
+        ];
+
+        $this->assertEquals($expected, $result);
+    }
+
+    /**
+     * @it resolveType allows resolving with type name
+     */
+    public function testResolveTypeAllowsResolvingWithTypeName()
+    {
+        $PetType = new InterfaceType([
+            'name' => 'Pet',
+            'resolveType' => function ($obj) {
+                return new Deferred(function () use ($obj) {
+                    if ($obj instanceof Dog) {
+                        return 'Dog';
+                    }
+                    if ($obj instanceof Cat) {
+                        return 'Cat';
+                    }
+                    return null;
+                });
+            },
+            'fields' => [
+                'name' => ['type' => Type::string()]
+            ]
+        ]);
+
+
+        $DogType = new ObjectType([
+            'name' => 'Dog',
+            'interfaces' => [$PetType],
+            'fields' => [
+                'name' => ['type' => Type::string()],
+                'woofs' => ['type' => Type::boolean()],
+            ]
+        ]);
+
+        $CatType = new ObjectType([
+            'name' => 'Cat',
+            'interfaces' => [$PetType],
+            'fields' => [
+                'name' => ['type' => Type::string()],
+                'meows' => ['type' => Type::boolean()],
+            ]
+        ]);
+
+        $schema = new Schema([
+            'query' => new ObjectType([
+                'name' => 'Query',
+                'fields' => [
+                    'pets' => [
+                        'type' => Type::listOf($PetType),
+                        'resolve' => function () {
+                            return [
+                                new Dog('Odie', true),
+                                new Cat('Garfield', false)
+                            ];
+                        }
+                    ]
+                ]
+            ]),
+            'types' => [$CatType, $DogType]
+        ]);
+
+        $query = '{
+      pets {
+        name
+        ... on Dog {
+          woofs
+        }
+        ... on Cat {
+          meows
+        }
+      }
+    }';
+
+        $result = GraphQL::execute($schema, $query);
+
+        $expected = [
+            'data' => [
+                'pets' => [
+                    ['name' => 'Odie', 'woofs' => true],
+                    ['name' => 'Garfield', 'meows' => false],
+                ]
+            ]
+        ];
+        $this->assertEquals($expected, $result);
+    }
+
+    /**
+     * @it resolveType can be caught
+     */
+    public function testResolveTypeCanBeCaught()
+    {
+
+        $PetType = new InterfaceType([
+            'name' => 'Pet',
+            'resolveType' => function () {
+                return new Deferred(function () {
+                    throw new \Exception('We are testing this error');
+                });
+            },
+            'fields' => [
+                'name' => ['type' => Type::string()]
+            ]
+        ]);
+
+        $DogType = new ObjectType([
+            'name' => 'Dog',
+            'interfaces' => [$PetType],
+            'fields' => [
+                'name' => ['type' => Type::string()],
+                'woofs' => ['type' => Type::boolean()],
+            ]
+        ]);
+
+        $CatType = new ObjectType([
+            'name' => 'Cat',
+            'interfaces' => [$PetType],
+            'fields' => [
+                'name' => ['type' => Type::string()],
+                'meows' => ['type' => Type::boolean()],
+            ]
+        ]);
+
+        $schema = new Schema([
+            'query' => new ObjectType([
+                'name' => 'Query',
+                'fields' => [
+                    'pets' => [
+                        'type' => Type::listOf($PetType),
+                        'resolve' => function () {
+                            return [
+                                new Dog('Odie', true),
+                                new Cat('Garfield', false)
+                            ];
+                        }
+                    ]
+                ]
+            ]),
+            'types' => [$CatType, $DogType]
+        ]);
+
+        $query = '{
+      pets {
+        name
+        ... on Dog {
+          woofs
+        }
+        ... on Cat {
+          meows
+        }
+      }
+    }';
+
+        $result = GraphQL::execute($schema, $query);
+
+        $expected = [
+            'data' => [
+                'pets' => [null, null]
+            ],
+            'errors' => [
+                [
+                    'message' => 'We are testing this error',
+                    'locations' => [['line' => 2, 'column' => 7]],
+                    'path' => ['pets', 0]
+                ],
+                [
+                    'message' => 'We are testing this error',
+                    'locations' => [['line' => 2, 'column' => 7]],
+                    'path' => ['pets', 1]
+                ]
+            ]
+        ];
+
+        $this->assertEquals($expected, $result);
+    }
+}