This updated and revised fourth edition sets out a Black Letter text of international humanitarian law accompanied by case analysis and extensive explanatory commentary. The book takes account of recent legal developments, such as the 2017 Nuclear Weapons Prohibition Treaty, as well as the ongoing debate on many old and new issues including the notion of direct participation in hostilities; air and missile warfare; military operations in outer space; military cyber operations; belligerent occupation; operational detention; and the protection of the environment in relation to armed conflict. The continuing need to consider borderline issues of the law of armed conflict as well as the interplay of international humanitarian law, human rights law, and other branches of international law is highlighted. Certain topics, such as the law of occupation, protection of the environment in relation to armed conflicts, humanitarian assistance, and human rights in armed conflict have been made more visible in separate chapters.