9306580 Nau This is the first year funding of a three-year continuing award. A primary goal of the proposed work is to define, analyze, and explicate features of the design of HTN planning systems. The proposed work will have the following benefits: It will correctly characterize HTN planning and provide a framework which accounts for many features used in existing planning systems, but not yet completely formalized (including reduction, critics, commitment choices and filter conditions). It will provide a clear understanding of the efficiency and expressivity of HTN planning, and how these compare with the efficiency and expressivity of other approaches to planning (such as planning with STRIPS-style operators). It will provide ways to determine whether a given set of methods and critics correctly describe the planning problem they were meant to solve. It will describe the characteristics of planning domains in which HTN planners can operate efficiently, and develop efficient planning systems for those domains.