In this paper we describe the use of behavior hierarchies based on "merging" two models of multi-layer architecture -- the supervenience model of (Spector, 1992) and the subsumption model of (Brooks, 1990). The behavior hierarchy approach allows us to use the robustness of reactivity in behavior design. It also encourages the design of modular behaviors that can be reused or more importantly recalibrated in different situations. We argue that behavior hierarchies extend our ability to design and program effective solutions that do not require any explicit planning. We also describe ongoing work in using this model for integrating planning and reacting This work is being used in support of an implemented system in which an autonomous mobile robot performs a vacuuming task.