Armed conflict and deprivation of liberty are inexorably linked. Deprivation of liberty by non-state armed groups is a consequence of the predominantly non-international character of contemporary armed conflicts. Regardless of the nature of the detaining authority or the overarching legality of its detention operations, deprivation of liberty may nonetheless have serious humanitarian implications for the individuals detained. Despite a need for humanitarian action, effective engagement is hampered by certain threshold obstacles, such as the perceived risk of the group’s legitimization. Since the formative work of the International Committee of the Red Cross (ICRC)’s founder, Henry Dunant, the ICRC has sought to overcome these obstacles. In doing so it draws upon its experience of humanitarian action in state detention, adapting it to the exigencies of armed groups and the peculiarities of their detention practice. Although not without setbacks, the ICRC retains a unique role in this regard and strives to ameliorate the treatment and conditions of detention of persons deprived of liberty by armed groups.