Hierarchical Planning is an Artificial Intelligence (AI) problem solving approach for a certain kind of planning problems -- the kind focusing on problem decomposition, where problems are step-wise refined into smaller and smaller ones until the problem is finally solved. A solution hereby is a sequence of actions that's executable in a given initial state (and a refinement of the initial compound tasks that needed to be refined). This form of hierarchical planning is usually referred to as Hierarchical Task Network (HTN) planning, but many variants and extensions exist.
This website provides some resources and links related to hierarchical planning that are hopefully useful to some.
Note that there's also a mailing list (with currently roughly 70 researchers subscribed) that's dedicated specifically to hierarchical planning. It is moderated (thus SPAM-free) and very low traffic as only content is allowed that's directly related to hierarchical planning. If you'd like to subscribe, please contact Pascal Bercher.