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Problem solving systems incorporating a truth or reason maintenance compo-nent have been developed for a number of dierent domains, including hypothetical reasoning, diagnosis, planning and circuit design. 1 Almost without exception, the problems addressed by these systems are well structured, that is, there exists a complete and consistent statement of the problem requirements when problem solving begins. In this paper we argue that RMS (and in particular assumption-based truth maintenance systems) can also be applied to ill structured problems. We describe a hybrid ATMS-blackboard architecture which has been employed in the development of a number of design support systems in dierent domains and brie y outline some of the questions raised by our experience with the architecture