This paper is about an approach to the flexible utilization of old plans called adaptive planning. An adaptive planner can take advantage of the details associated with specific plans, while still maintaining the flexibility of a planner that works from general plans. Key elements in the theory of adaptive planning are its treatment of background knowledge and the introduction of a notion of planning by situation matching. This paper will discuss a theory of adaptive planning as it applies to the domain of common-sense planning. The theory of adaptive planning has been embodied in a computer program called PLEXUS.