diff --git a/types/joi/index.d.ts b/types/joi/index.d.ts
index fb3547b3e4..0772c82fbe 100644
--- a/types/joi/index.d.ts
+++ b/types/joi/index.d.ts
@@ -2,11 +2,11 @@
 // Project: https://github.com/hapijs/joi
 // Definitions by: Bart van der Schoor <https://github.com/Bartvds>, Laurence Dougal Myers <https://github.com/laurence-myers>, Christopher Glantschnig <https://github.com/cglantschnig>, David Broder-Rodgers <https://github.com/DavidBR-SW>, Gael Magnan de Bornier <https://github.com/GaelMagnan>, Rytis Alekna <https://github.com/ralekna>, Pavel Ivanov <https://github.com/schfkt>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
+// TypeScript Version: 2.4
 
 // TODO express type of Schema in a type-parameter (.default, .valid, .example etc)
 
 
-
 export interface ValidationOptions {
     /**
      * when true, stops validation on the first error, otherwise returns all the errors found. Defaults to true.
@@ -337,6 +337,25 @@ export interface AnySchema<T extends AnySchema<Schema>> {
      * option has been set to `false`).
      */
     error?(err: Error | ValidationErrorFunction): T;
+
+    /**
+     * Creates a joi error object.
+     * Used in conjunction with custom rules.
+     * @param ruleName - the rule name to create the error for.
+     * @param context - must contain `v` eg. the value that was validated, all other properties
+     *  are available in the `language` templates.
+     * @param state - should the context passed into the `validate` function in a
+     *  custom rule
+     * @param options - should the context passed into the `validate` function in a
+     *  custom rule
+     */
+    createError(ruleName: string, context: {
+        v: any;
+        [key: string]: any;
+    }, state: State, options: ValidationOptions): Err;
+}
+
+export interface State {
 }
 
 export interface BooleanSchema extends AnySchema<BooleanSchema> {
@@ -825,20 +844,26 @@ export interface Terms {
 export interface Rules {
     name: string;
     params?: ObjectSchema | { [key: string]: Schema };
-    setup?: Function;
-    validate?: Function;
+    setup?: (this: AnySchema<AnySchema<Schema>>, description: { [key: string]: any }) => AnySchema<AnySchema<Schema>> | void;
+    validate?: (this: AnySchema<AnySchema<Schema>>, params: { [key: string]: any }, value: any, state: State, options: ValidationOptions) => Err | undefined;
     description?: string | Function;
 }
 
 export interface Extension {
     name: string;
     base?: Schema;
-    pre?: Function;
-    language?: {};
-    describe?: Function;
+    pre?: (this: AnySchema<AnySchema<Schema>>, params: { [key: string]: any }, value: any, state: State, options: ValidationOptions) => Err | void;
+    language?: {
+        [key: string]: string;
+    },
+    describe?: (this: AnySchema<AnySchema<Schema>>, description: any) => any;
     rules?: Rules[];
 }
 
+export interface Err {
+    toString(): string;
+}
+
 // --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- ---
 
 /**
diff --git a/types/joi/joi-tests.ts b/types/joi/joi-tests.ts
index a0684724b7..7bfb712247 100644
--- a/types/joi/joi-tests.ts
+++ b/types/joi/joi-tests.ts
@@ -886,6 +886,35 @@ schema = Joi.reach(schema, '');
 
 const Joi2 = Joi.extend({ name: '', base: schema });
 
+const Joi3 = Joi.extend({
+    base: Joi.string(),
+    name: 'string',
+    language: {
+        asd: 'must be exactly asd(f)',
+    },
+    pre (value, state, options) {
+    },
+    describe (description) {
+    },
+    rules: [
+        {
+            name: 'asd',
+            params: {
+                allowF: Joi.boolean().default(false),
+            },
+            setup (params) {
+                const fIsAllowed = params.allowF;
+            },
+            validate (params, value, state, options) {
+                if (value === 'asd' && (params.allowF && value === 'asdf')) {
+                    return value;
+                }
+                return this.createError('asd', { v: value }, state, options);
+            },
+        },
+    ],
+});
+
 // --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- ---
 
 schema = Joi.allow(x, x);