So far, we have described the space of mental situations constructed by STRIPS. This space is, in fact, a tree that has been specified by the initial mental situation, the set of appropriate mental actions for a mental situation, and the effects of mental actions on mental situations. We describe now the search strategy according to which STRIPS explores such a tree of mental situations in order to find a solution to a planning problem, and the heuristics it uses for selecting actions and detecting dead-ends during the searching process.