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.