A forward projection is a useful concept for characterizing the behavior of plans during execution. Before uncertainties were considered, a plan was executed exactly as expected. When a sequence of actions was applied to an initial state, the resulting sequence of states could be computed using the state transition equation. Now that the state transitions are unpredictable, we would like to imagine what states are possible several stages into the future. In the case of nondeterministic uncertainty, this involves computing a set of possible future states, given a current state and plan. In the probabilistic case, a probability distribution over states is computed instead.