▪ Abstract  How do terrorists and terrorist groups make decisions? And what influence do terrorist decision-making styles have on the course of a terrorist campaign? Efforts to answer these questions have centered on three sets of theories. In order of generality, these are (a) strategic theories, in which the decision to employ terrorism and related forms of political violence is considered to be an instrumental choice; (b) organizational theories, in which the sources of violence are found in the internal dynamics of the terrorist group itself; and (c) psychological theories, in which the decision to employ terrorism is explained within the framework of individual psychology. Most observers agree that these lines of inquiry are not mutually exclusive, but each offers a distinctive approach to terrorist decision making. This essay examines each of these theoretical approaches in turn. It concludes with a brief discussion of the decision-making constraints that help shape terrorist life cycles.