diff --git a/when/when.d.ts b/when/when.d.ts
index a9e9d483d..c90478c10 100644
--- a/when/when.d.ts
+++ b/when/when.d.ts
@@ -7,6 +7,10 @@ declare function When<T>(value: When.Promise<T>): When.Promise<T>;
 declare function When<T>(value: When.Thenable<T>): When.Promise<T>;
 declare function When<T>(value: T): When.Promise<T>;
 
+declare function When<T, U>(value: When.Promise<T>, transform: (val: T) => U): When.Promise<U>;
+declare function When<T, U>(value: When.Thenable<T>, transform: (val: T) => U): When.Promise<U>;
+declare function When<T, U>(value: T, transform: (val: T) => U): When.Promise<U>;
+
 declare module When {
 
     /**