The past decades have seen a remarkable development of military operations both within the United Nations collective security system and in other international settings. While traditional forms of military operations have been maintained and further developed, there have also been substantive developments, responding to new challenges for international security, the specific requirements of international and multinational cooperation, and legal regulation. Treaty law, customary law, and best practice relevant for military operations derive from various branches of international law which have to be applied in context. Cooperation between States and International Organizations has brought about a progressive development of applicable rules, and a requirement for legal control both at the national and international level. At the same time, the correct application of legal rules and best practice has become one of the benchmarks for the assessment of military operations and failure to meet appropriate standards can have significant military and political, as well as legal, implications. This makes the identification and correct application of these rules of crucial importance in the planning and conduct of all types of military operations. The absence of an all-encompassing set of regulations and the need to find specific solutions for tasks characterized by an interdependence of efforts have made a reassessment of this important part of international law both a timely and topical task. Renowned international lawyers have joined together in this project to offer their insight in the relevant principles and provisions. They address important rules for enforcement, peace enforcement, and peace operations, as well as for other military operations conducted within the context of self-defence and other possible legal bases for the use of force.