diff --git a/README.md b/README.md
index 9036037..a1cc75e 100644
--- a/README.md
+++ b/README.md
@@ -559,6 +559,24 @@ $queryDepth->setMaxQueryDepth($maxQueryDepth = 10);
 GraphQL::execute(/*...*/);
 ```
 
+#### Disabling Introspection
+
+This is a PHP port of [graphql-disable-introspection](https://github.com/helfer/graphql-disable-introspection).
+The rule prohibits queries that contain `__type` or `__schema` fields.
+
+This document validator rule is disabled by default. Here an example to enable it:
+
+```php
+use GraphQL\GraphQL;
+use GraphQL\Validator\Rules\DisableIntrospection;
+
+/** @var \GraphQL\Validator\Rules\DisableIntrospection $disableIntrospection */
+$disableIntrospection = DocumentValidator::getRule('DisableIntrospection');
+$disableIntrospection->setEnabled(DisableIntrospection::ENABLED);
+
+GraphQL::execute(/*...*/);
+```
+
 ### More Examples
 Make sure to check [tests](https://github.com/webonyx/graphql-php/tree/master/tests) for more usage examples.
 
diff --git a/src/GraphQL.php b/src/GraphQL.php
index d0d3ec6..1572c82 100644
--- a/src/GraphQL.php
+++ b/src/GraphQL.php
@@ -42,9 +42,9 @@ class GraphQL
     /**
      * @param Schema $schema
      * @param string|DocumentNode $requestString
-     * @param null $rootValue
-     * @param null $variableValues
-     * @param null $operationName
+     * @param mixed $rootValue
+     * @param array|null $variableValues
+     * @param string|null $operationName
      * @return ExecutionResult|Promise
      */
     public static function executeAndReturnResult(Schema $schema, $requestString, $rootValue = null, $contextValue = null, $variableValues = null, $operationName = null)
diff --git a/src/Validator/DocumentValidator.php b/src/Validator/DocumentValidator.php
index 540765d..837080a 100644
--- a/src/Validator/DocumentValidator.php
+++ b/src/Validator/DocumentValidator.php
@@ -25,6 +25,7 @@ use GraphQL\Utils;
 use GraphQL\Utils\TypeInfo;
 use GraphQL\Validator\Rules\ArgumentsOfCorrectType;
 use GraphQL\Validator\Rules\DefaultValuesOfCorrectType;
+use GraphQL\Validator\Rules\DisableIntrospection;
 use GraphQL\Validator\Rules\FieldsOnCorrectType;
 use GraphQL\Validator\Rules\FragmentsOnCompositeTypes;
 use GraphQL\Validator\Rules\KnownArgumentNames;
@@ -100,6 +101,7 @@ class DocumentValidator
                 'UniqueInputFieldNames' => new UniqueInputFieldNames(),
 
                 // Query Security
+                'DisableIntrospection' => new DisableIntrospection(DisableIntrospection::DISABLED), // DEFAULT DISABLED
                 'QueryDepth' => new QueryDepth(QueryDepth::DISABLED), // default disabled
                 'QueryComplexity' => new QueryComplexity(QueryComplexity::DISABLED), // default disabled
             ];
diff --git a/src/Validator/Rules/DisableIntrospection.php b/src/Validator/Rules/DisableIntrospection.php
new file mode 100644
index 0000000..f820375
--- /dev/null
+++ b/src/Validator/Rules/DisableIntrospection.php
@@ -0,0 +1,50 @@
+<?php
+namespace GraphQL\Validator\Rules;
+
+use GraphQL\Error\Error;
+use GraphQL\Language\AST\FieldNode;
+use GraphQL\Language\AST\NodeKind;
+use GraphQL\Validator\ValidationContext;
+
+class DisableIntrospection extends AbstractQuerySecurity
+{
+    const ENABLED = 1;
+    private $isEnabled;
+
+    public function __construct($enabled)
+    {
+        $this->setEnabled($enabled);
+    }
+
+    public function setEnabled($enabled)
+    {
+        $this->isEnabled = $enabled;
+    }
+
+    static function introspectionDisabledMessage()
+    {
+        return 'GraphQL introspection is not allowed, but the query contained __schema or __type';
+    }
+
+    protected function isEnabled()
+    {
+        return $this->isEnabled !== static::DISABLED;
+    }
+
+    public function __invoke(ValidationContext $context)
+    {
+        return $this->invokeIfNeeded(
+            $context,
+            [
+                NodeKind::FIELD => function (FieldNode $node) use ($context) {
+                    if ($node->name->value === '__type' || $node->name->value === '__schema') {
+                        $context->reportError(new Error(
+                            static::introspectionDisabledMessage(),
+                            [$node]
+                        ));
+                    }
+                }
+            ]
+        );
+    }
+}
diff --git a/tests/Validator/DisableIntrospectionTest.php b/tests/Validator/DisableIntrospectionTest.php
new file mode 100644
index 0000000..88d771c
--- /dev/null
+++ b/tests/Validator/DisableIntrospectionTest.php
@@ -0,0 +1,128 @@
+<?php
+namespace GraphQL\Tests\Validator;
+
+use GraphQL\Error\FormattedError;
+use GraphQL\Language\SourceLocation;
+use GraphQL\Validator\Rules\DisableIntrospection;
+
+class DisableIntrospectionTest extends TestCase
+{
+    // Validate: Disable Introspection
+
+    /**
+     * @it fails if the query contains __schema
+     */
+    public function testQueryContainsSchema()
+    {
+        $this->expectFailsRule(new DisableIntrospection(DisableIntrospection::ENABLED), '
+      query { 
+        __schema {
+          queryType {
+            name
+          }
+        }
+      }
+        ',
+            [$this->error(3, 9)] 
+        );
+    }
+    
+    /**
+     * @it fails if the query contains __type
+     */
+    public function testQueryContainsType()
+    {
+        $this->expectFailsRule(new DisableIntrospection(DisableIntrospection::ENABLED), '
+      query { 
+        __type(
+          name: "Query"
+        ){
+          name
+        }
+      }
+        ',
+            [$this->error(3, 9)]
+        );
+    }
+
+    /**
+     * @it does not fail on a query that does not contain __type
+     */
+    public function testValidQuery()
+    {
+        $this->expectPassesRule(new DisableIntrospection(DisableIntrospection::ENABLED), '
+      query {
+        user {
+          name
+          email
+          friends {
+            name
+          }
+        }
+      }
+        ');
+    }
+
+    /**
+     * @it does not fail when not enabled
+     */
+    public function testQueryWhenDisabled()
+    {
+        $this->expectPassesRule(new DisableIntrospection(DisableIntrospection::DISABLED), '
+      query { 
+        __type(
+          name: "Query"
+        ){
+          name
+        }
+      }
+        ');
+    }
+
+    /**
+     * @it has a public interface for enabeling the rule
+     */
+    public function testPublicEnableInterface()
+    {
+        $disableIntrospection = new DisableIntrospection(DisableIntrospection::DISABLED);
+        $disableIntrospection->setEnabled(DisableIntrospection::ENABLED);
+        $this->expectFailsRule($disableIntrospection, '
+      query { 
+        __type(
+          name: "Query"
+        ){
+          name
+        }
+      }
+        ',
+            [$this->error(3, 9)]
+        );
+    }
+
+    /**
+     * @it has a public interface for disableing the rule
+     */
+    public function testPublicDisableInterface()
+    {
+        $disableIntrospection = new DisableIntrospection(DisableIntrospection::ENABLED);
+        $disableIntrospection->setEnabled(DisableIntrospection::DISABLED);
+        $this->expectPassesRule($disableIntrospection, '
+      query { 
+        __type(
+          name: "Query"
+        ){
+          name
+        }
+      }
+        ');
+    }
+
+
+    private function error($line, $column)
+    {
+        return FormattedError::create(
+            DisableIntrospection::introspectionDisabledMessage(),
+            [ new SourceLocation($line, $column) ]
+        );
+    }
+}