Worldwide, around 600 armed groups have the capacity to cause violence of humanitarian concern – and more than 100 of those can, as a matter of international humanitarian law, be considered parties to armed conflicts. The prevalence of these groups and their corresponding ability to project power – through both exercising violence and providing services to those under their control – make them an ongoing issue of humanitarian concern. This edition explores complex questions of governance and international law raised by the existence and operations of non-state armed groups all around the world, asking what laws they are bound by, how to engage with these groups, and what duties they owe those under their control. Included in the edition as well is a collection of selected articles on a variety of cutting-edge topics.