With most contemporary armed conflicts being of a non-international nature, non-State armed groups (NSAGs) play a prominent role in the factual reality regulated by international humanitarian law (IHL). While it is widely recognised that NSAGs have obligations under IHL applicable in non-international armed conflicts (NIACs), their role in the creation of this body of law remains highly controversial. Accepting that the capacity of NSAGs to contribute to the development of IHL rests on the consent of States, the authors demonstrate that NSAGs have thus far only been granted limited capacity to do so. Yet, there are feasible avenues for increasing the participation of NSAGs in the creation of IHL rules applicable in NIACs, which might contribute to enhanced compliance with IHL by NSAGs and lead to a more realistic and conceptually coherent legal regime. In addition, NSAGs can and do play an important role in the interpretation of IHL rules applicable in NIACs. The process of interpretation provides an opportunity for NSAGs to influence the legal content of these rules without directly challenging the primacy of States as international law-makers.