Terrorism, a rare and extreme form of political behavior, is dependent on the motivations of the small numbers who practice it. Terrorism is based on systematic and purposive violence, designed to influence the political choices of other actors more than to inflict casualties or material destruction. Psychology helps answer questions of why the individual becomes a terrorist, how terrorist groups are formed and act, and why publics and governments react with alarm despite the minor physical menace of terrorism. Political contexts include democracies as well as authoritarian regimes and states ranging from strong to weak in coercive capability and political stability. Terrorist organizations are likely to be composed of people whose need is the group and others whose goals are to change their social and political environment. The political effectiveness of terrorism is importantly determined by the psychological effects of violence on audiences.