During armed conflict, non-State armed groups frequently establish their own judicial systems to resolve disputes, impose penal sanctions and implement social control. Examples such Hamas in Gaza, Rojava in Syria, or the 'People's Republics' in Eastern Ukraine demonstrate that this aspect of 'rebel governance' has become increasingly common. How can or should international law regulate the establishment of courts, conduct of trials and passing of penal sanctions by insurgent movements that challenge the judicial monopoly of states? Based on an in-depth doctrinal analysis, this study demonstrates that the administration of criminal justice by insurgents is not inherently illegal or illegitimate, and explains how to measure the conduct of armed groups against clear legal standards. Drawing on a broad range of real-world examples, this study makes a vital contribution to the law applicable in armed conflict.