Under international law, armed groups parties to non-international armed conflicts are not absolutely prohibited from detaining individuals. Rather, under international human rights law and international humanitarian law, they are addressed by the prohibition of arbitrary deprivation of liberty. This prohibition requires that any detention be carried out pursuant to a legal basis and for legitimate reasons, and that the possibility to challenge the detention be granted to any person deprived of liberty. These standards need to be interpreted so as to make it possible for armed groups to comply with them and to detain individuals in a non-arbitrary way. In this thesis, these standards are specifically interpreted and applied to the detentions of three categories of individuals: (1) armed group members detained by their own group for disciplinary or criminal reasons, in relation to internal or international offenses; (2) individuals subjected to security detentions for reasons related to the armed conflict; (3) individuals other than the armed group’s own members who are detained in connection with criminal proceedings for common or international crimes. It is in the interest of these individuals that risks of arbitrary detention by armed groups should be minimized by understanding what safeguards exist and how armed groups can comply with them. The thesis concludes by proposing ten basic principles which should inform any detention by armed groups, and which are at the same time sufficiently protective for detainees but realistic for armed groups to comply with.