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.