Are there objective criteria that we can use to discern if an act of violence constitutes terrorism, or is such labeling always a subjective and political decision? Wherein lies the boundary between domestic versus international terrori and is that a meaningful distinction to make? How do individuals get radicalized, and how do they reach the point of committing violent acts? In this chapter, we tackle these questions (and others) and the issue of terrorism in international security. There are no easy, agreed upon answers to most of them, and terrorism continues to be a highly contested and politically charged concept, while constituting a very real and pressing security threat in many countries around the world. But that is even more reason to look closely at the controversies surrounding its definition, its historical evolution and patterns, and its contemporary manifestations in the twenty-first century as well as approaches to countering terrorism and attempts at international cooperation.

This chapter explores terrorism, terrorist groups, and potential solutions to terrorism. It discusses conceptual and legal definitions of terrorism. It then provides historical context, describing terrorist waves driven by ideologies such as anarchism, anti-colonialism, New Left perspectives, and various religions. The chapter looks at who becomes a terrorist, taking account of theories of grievance, radicalization, recruitment by terrorist groups, lone wolf terrorism, and foreign fighter recruitment. It considers relationships between regime type and terrorism, examining why democracies, anocracies, and autocracies respectively might be more or less likely to attract terrorist violence, and exploring issues such as media coverage, grievance, and repression. Terrorist tactics and how these might diffuse across groups are discussed, as are terrorist group cooperation and state sponsorship of terrorism. The chapter also looks at the challenges of crafting effective counterterrorism policy, with a focus on deradicalization. It then applies many of these concepts to a quantitative study on whether targeting terrorist group leaders helps stop terrorism, and a case study of al-Qaeda.