Intentions, an integral part of the mental state of an agent, play an important role in determining the behavior of rational agents as they seek to attain their goals. In this paper, a formalization of intentions based on a branching-time possible-worlds model is presented. It is shown how the formalism realizes many of the important elements of Bratman's theory of intention. In particular, the notion of intention developed here has equal status with the notions of belief and desire.