This article tackles the question of whether international law entitles armed groups to detain people, as well as the separate question of what international law obligations bind the armed group when persons are de-tained. The focus is on the obligations that relate to the right to challenge the basis for any such detention, although some attention is given to issues of fair trial and the question of punishment. The last part of the article briefly considers the legal framework governing responsibility for States and those that assist armed groups. State responsibility questions relating to attribution and assistance are considered, as are the separate rules which would determine the criminal responsibility of accomplices who could be prosecuted.