Since 9/11, terrorism has been widely perceived as the foremost threat to the United States, its allies, and the broader international community. Political scientists have historically paid little attention to the study of terrorism and counterterrorism; in the subfield of international relations (IR), the focus of research of the dominant realist tradition was on great power politics, not on substate violence. In the post-9/11 world, IR scholars have begun to show interest in the causes and consequences of terrorism. Studies undertaken since October 2001 have been increasingly quantitative, employing a mixture of descriptive and inferential statistical analyses. Yet this heightened scholarly attention has yielded few uncontested insights. Fundamental methodological, empirical, and theoretical questions about terrorism have become the subject of intense discussions. The definition of terrorism in particular remains problematic. Scholars also debate over the virtues of large-n studies versus case studies, the accuracy of terrorism events data, and al-Qaida’s place within the history of terrorism. In the case of counterterrorism, much of the literature has followed policy trends rather than developing empirically grounded theories. Two strands of counterterrorism literature are country case studies and discussions on the relative merits of different policy instruments. There has been increased interest in systematic studies of counterterrorism effectiveness and the nascent development of theories on the sources of counterterrorist policies in recent years, which raises the possibility for theoretically informed and methodologically aware debates in the study of state responses to terrorism.