Abstract We show how to translate HTN domain descriptions (if they satisfy certain restrictions) into PDDL so that they can be used by classical planners. We provide correctness results for our translation algorithm, and show that it runs in linear time and space. We also show that even small and incomplete amounts of HTN knowledge, when translated into PDDL using our algorithm, can greatly improve a classical planner’s performance. In experiments on several thousand randomly generated problems in three different planning domains, such knowledge speeded up the well-known FastForward planner by several orders of magnitude, and enabled it to solve much larger problems than it could otherwise solve