The accountability of armed non-state actors is a neglected field of international law, overtaken by the regimes of state responsibility and individual criminal accountability as well as fears of legitimacy. Yet armed non-state actors are important players in the international arena and their activities have significant repercussions. This book focuses on their obligations and accountability when they do not function as state agents, regardless of the existence or extent of accountability of their individual members. The author claims that their distinct features lead to their classification into three different types: de facto entities, armed non-state actors in control of territory, and common article 3 armed non-state actors. The mechanisms that trigger the applicability of humanitarian and human rights law regimes are examined in detail as well as the framework of obligations. In both cases, the author argues that armed non-state actors should not be treated as entering international law and process exclusively through the state. The study concludes by focussing on their accountability in international humanitarian and human rights law and, more specifically, to the rules of attribution, remedies and reparations for violations of their primary obligations.