We describe an incremental and interactive approach to fixture and process planning for a concurrent design system. A fixture agent interacts with a process planner and a geometric reasoning module to develop plans for clamping machined parts. Interactions are accomplished via messages and shared representations of constraints such as ordering relationships among proceess and setups. The approach is incremental because process and fixture planning are done step-by-step as a design eveloves, reusing previous results for fast response. The reuse of previous results is faciliated by explicit representations of dependencies among process plan steps, setups and design features. We present some underlying design principles for modules such as the fixture agent, that function as part of cooperating team for concurrent design and we conclude with a discussion of extensions to permit more sophisticated forms of cooperation in the future.