Additional Protocol I to the Geneva Conventions of 1949 requires that legal advisers be made available to military commanders, particularly during hostilities. This treaty stipulation was quite innovative in 1977, but it has achieved widespread implementation, even among non-Contracting Parties. It is noteworthy that the United States—which objects to numerous provisions of Additional Protocol I—does not dissent from the article requiring legal advisers. A study of the practice of States, made by the International Committee of the Red Cross, confirms that the norm requiring that legal advisers be made available to advise military commanders in time of armed conflict currently reflects customary international law. This essay examines how the requirement is implemented by States and how States view the specific role of the legal adviser, their relationship to the military commander, their training (as well the commander’s training), and responsibility for faulty advice.