This chapter examines the grounds upon which persons may be detained by States in the context of an armed conflict, as well as the requirements for supervision of the lawfulness of such detention. It considers the legal basis for deprivation of liberty in both international armed conflicts (IAC) - looking at the authority to detain prisoners of war (POWs) and civilians - and non-international armed conflicts (NIAC). While States’ authority to detain in an IAC can be found in Geneva Conventions III and IV and Additional Protocol I, international humanitarian law (IHL) treaties are silent on grounds or procedure for internment in the context of a NIAC. The author refutes the position taken in the ICRC’s Opinion Paper on internment in armed conflict, namely that an implied power to intern in a NIAC can be inferred from Common Article 3 to the Geneva Conventions and Additional Protocol II. Drawing attention on eight different arguments, the author seeks to demonstrate that IHL does not provide a legal authority to deprive a person of liberty in NIACs. Instead, detention in a NIAC must comply with international human rights Law, including the right to habeas corpus, which protects against arbitrary arrest or detention. The possibility to extend the right to habeas corpus to POWs is also discussed.