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A search algorithm whose OR nodes represent controllable choices and whose AND nodes require solutions for every possible outcome of a chosen action. In nondeterministic planning, a successful OR choice needs successful continuations for all its AND children. In contingent planning, those children can represent different observations and updated belief states. Cycle handling must match the required solution concept: ignoring a repeated state is not a proof that the goal will inevitably be reached.

A search problem in which the available observations need not identify the current physical state. The controller must choose the same action for physical states that its observed history cannot distinguish. A belief state summarizes those possibilities; contingent planning selects future actions according to subsequent observations.