Armed groups operating beyond the state have become the most important actors in most contemporary wars and violent conflicts, from Iraq and Afghanistan to Colombia and Somalia. They come in a dizzying array of forms: some informally linked to the state and state power, others in opposition to the state; some pursuing classic political goals, others primarily predatory and large-scale criminal enterprises. All groups, however, challenge the state's Weberian monopoly of the legitimate use of force, yet their origins, evolution, violent dynamics, and relations with state power are poorly understood. This interdisciplinary collection includes both conceptual and empirical studies of contemporary armed groups, examining cases in Latin America, Asia and Africa. It brings sociological, political economy, and ethnographic approaches to bear on larger questions including armed groups and the changing nature of warfare, the economic dimensions of their activities, and means of engagement with armed actors. It both broadens and sharpens our understanding of how force and violence are used in today's contemporary armed conflicts. This book was published as a special issue of Contemporary Security Policy.