The book serves as a companion to three other volumes published by Cambridge University Press, dealing respectively with the jus ad bellum, the law of belligerent occupation, and non-international armed conflicts. It is devoted to the core of the jus in bello - that is, the conduct of hostilities on land, at sea and in the air in inter-State armed conflicts - analyzed against the background of customary international law and treaties in force. The book deals with both means and methods of modern warfare. It addresses issues of general non-combatant protection, the principle of proportionality in collateral damage to civilians, and special protection, especially of the environment and cultural property. It also considers the relevant dimensions of international criminal law and deals with controversial matters such as unlawful combatancy, direct participation of civilians in hostilities and the use of 'human shields'. Case law and legal literature are cited throughout.