This thesis examines the legal significance of armed groups' practice in the formation and identification of customary international humanitarian law (IHL) applicable to non-international armed conflicts. Although the issue has been widely discussed since the publication of the ICRC's 2005 Customary IHL Study, the topic has not been examined in a comprehensive and systematic manner to date. This thesis fills this gap. It does so, first, by analysing the matter from a theoretical perspective, paying particular attention to the nature of customary law as a source of law and exploring whether this analysis can shed light on the ratione personae scope of its potential contributors. To complement the theoretical analysis, the thesis assesses whether it is conceptually and practically feasible to accommodate the contribution of armed groups to customary iIHL within the state-centric reality of international law.