This article explores the authority to detain individuals on the battlefield under both international and domestic law. Both treaty law and Customary International Law (CIL) provide reasonably clear authority to detain individuals during the conduct of armed conflict. Domestically, the authority is more uncertain. Given the constitutional allocation of war powers generally, and the absence of an express allocation of detention authority specifically, the existence and parameters of powers in this area remain fiercely contested issues. Broadly stated, Presidents have historically exercised detention authority without congressional authorization, and Congress has largely acquiesced. This is not, however, universally true.