While detention by armed opposition groups in non-international armed conflict is a reality that is foreseen and not prohibited by international humanitarian law, the grounds upon which it may take place are not defined. This article looks more closely at the customary international humanitarian law prohibition on arbitrary deprivation of liberty, and how it can apply to armed opposition groups in a manner that makes compliance realistic. It focuses on the legal bases upon which armed opposition groups may detain persons who are taken into custody in order to remove them from hostilities or for security purposes. An approach to detention by armed opposition groups based on the principles of international humanitarian law applicable to international armed conflicts is explored and its limitations defined.