Multilateral operations - most commonly peacekeeping operations under UN command and control, via regional organizations, or some combination of the two - have traditionally focused on monitoring and reporting on ceasefires or peace agreements. Today, they are given increasingly robust mandates, going so far as to authorize the use of force under certain circumstances. This issue of the Review examines a number of legal and humanitarian challenges related to multinational operations, including: the applicability of international humanitarian law and other legal regimes, the legal classification of situations involving multinational forces, the status of peacekeepers, interoperability difficulties, detention, attribution of responsibility, as well as humanitarian engagement with multinational forces.