Mobile robot architectures have been based on many different design principles: AI, control theory, hierarchical organization, etc. Brooks argues for a `subsumption' approach, based on layers of very simple, real-time computations. The CMU Navlab project takes a more pragmatic approach. The bottom layer is real-time, based on local coordinates, with no high- level models or central data structures to be bottlenecks. But the architectural tools developed for the Navlab also provide hooks for a higher level, based in world coordinates and using AI planning, to control the lower layer.