The potential use of nuclear weapons has long been a global concern. This article highlights the principal rules of international humanitarian law (IHL) governing the conduct of hostilities applicable to nuclear weapons, and the issues and concerns that would arise were such weapons ever to be used again, in particular the severe and extensive consequences for civilians, civilian objects, combatants and the environment. In recent years, increased attention has been paid to the humanitarian consequences of nuclear weapons. Based on what has been learned from extensive research on the humanitarian and environmental effects of nuclear weapons since they were first used in 1945, and the accompanying implications for IHL, it seems appropriate to conclude that the use of nuclear weapons in or near a populated area would amount to an indiscriminate attack and that there should also be a presumption of illegality with regard to the use of nuclear weapons outside such areas.