Deprivation of liberty - detention - is a common and lawful occurrence in armed conflict that is governed by a large number of provisions of international humanitarian law (IHL). Like other bodies of law, IHL prohibits arbitrary detention. This PDF outlines the basic concepts and rules related to detention in both international and non-international armed conflicts with, subsequently, a particular focus on internment (i.e. detention for security reasons in situations of armed conflict.) The similarities and differences between IHL and the corresponding rules of international human rights law are also addressed where relevant.