The design of a hierarchical planning system in which each level operates in parallel and communicates asynchronously is presented. It is shown that this Parallel Hierarchical Replanner is both reactive, and as close to optimal over all information in the state space as is possible given finite computational power. A comparison with three other hierarchical methods is presented, which demonstrates that for scenarios in which the time taken to achieve a mission goal is of greater importance than the cost incurred, this approach has better performance than related methods in the literature.