This book presents nine up-to-date chapters on key aspects of terrorist groups by leading contributors in the study of terrorism. The chapters focus on the study of terrorist groups, their interface with targeted governments as well as ways to counter politically motivated terrorist attacks. To augment the efficacy of counterterrorism, governments must understand how terrorist groups form, recruit members, gain support, and choose targets. Additionally, governments must be aware of interactions within and among terrorist groups to allow governments to hone the practice of counterterrorism. This book addresses many of these topics. In particular, the volume casts light on group formation during 1860–1969 and 1970–2016, respectively. From different vantages, terrorist recruitment and group support are investigated. Other topics include the role of terrorist groups’ ideologies and goals, determinants of home-base attacks, groups’ response to government countermeasures, intergroup interactions as well as conflicts. On Terrorist Groups will be of interest to students and researchers of Terrorism, Political Violence, Security and Intelligence, Conflict Studies, and Political Science in general. This book was originally published as a special issue of Defence and Peace Economics.