The Copenhagen Principles on the Handling of Detainees in International Military Operations were released in October 2012 after a five-year long process involving states and certain organizations. The Principles address a number of issues concerning the handling and transfer of detainees. They apply in military operations conducted by states abroad in the context of non-international armed conflicts and peace operations. This article focuses on those principles that address the procedural regulation of internment (ie preventive, security detention), as it is here that the current law is particularly unclear. On the one hand, the treaty provisions applicable in non-international armed conflicts contain no rules on the procedural regulation of internment, in comparison with the law of international armed conflict. On the other hand, the relevant rules under international human rights law (IHRL) appear derogable in such situations. This article demonstrates that the approach taken to this issue in the Copenhagen Principles is one which essentially draws on the procedural rules applicable to civilian internment in the international armed conflicts. These rules adopt standards that are lower than those under IHRL. Reference is then made to other recent practice, which illustrates that the Copenhagen Principles do not apply in a legal vacuum. In particular, two recent judicial developments highlight the continued relevance of human rights law and domestic law, respectively, in regulating detention operations in the context of international military operations. Compliance with the Copenhagen Principles may not, therefore, be sufficient for detention to be lawful.