This paper discusses three main areas of controversy relating to detention in the context of multinational operations: the relationship between international humanitarian law and human rights law; the principle of legality in the context of relying on United Nations Security Council resolutions as a justification for taking detainees; and the transfer of detainees where there is, for example, a substantial risk of torture or cruel, inhuman or degrading treatment or punishment. The paper then considers how the Copenhagen Principles address these issues. Keywords: detention, Copenhagen Principles, international humanitarian law, IHL, international human rights law, IHRL, the principle of legality, Security Council resolutions, transfers of detainees, monitoring the treatment of detainees.