Recent work in planning has rejected the assumption of a closed, stable world, and the associated paradigm of exhaustive preplanning, which encounters serious problems trying to plan in a world where that assumption does not hold. Several alternative strategies have been proposed, responding to these new problems in a variety of ways. W e review this spectrum, finding the various approaches in part incompatible but not bereft of some common themes and complementary strengths. W e suggest factors in the application domain which should influence the appropriate mix, and describe the T R U C K E R project to illustrate some of the problems and benefits in implementing such a mix.
