Social science research on terrorism has grown rapidly in recent years, aided by social and legal studies. In this review, we examine research on the causes of terrorism and the effectiveness of strategies for countering it. We define terrorism as the threatened or actual use of illegal force directed against civilian targets by nonstate actors in order to attain a political goal through fear, coercion, or intimidation. Our review of causes is divided by level of analysis into sections on individual-, group-, and macro-level explanations. Our evaluation of counterterrorism strategies includes reviews of legal, criminal justice, military, and legitimacy campaigns. Psychopathological approaches have been largely discredited; however, evidence suggests that certain experiences, attitudes, and behaviors are overrepresented among terrorists. The potential impact of ideology, leadership, popular support, Diasporic communities, and socialization on group dynamics all provide fertile areas for additional research. Terrorism feeds on the ability of groups to portray governments and their agents as illegitimate. There is also evidence for high rates of terrorism in countries that are in democratic transition or are described as failed states. Research on connections between religious motives and terrorism is mixed. Although research on the efficacy of counterterrorism methods has grown rapidly in recent years, it is still rarely evaluated with strong empirical methods. All of these conclusions are qualified by the fact that both individual- and group-level empirical data on terrorism are in short supply.