This article argues that humanitarian intervention to prevent the mass slaughter by a state of its own citizens is not only a morally but a legally justifiable act under current norms of international law. The first section of the article discusses the traditional international legal rules concerning the doctrine of humanitarian intervention and their relevance to contemporary law. The second section analyzes the effect of the advent of the United Nations Charter on the legality of humanitarian intervention. Drawing on state practice and the opinion of the international legal community, the third section argues that the emergence of a post-Charter doctrine of humanitarian intervention now constitutes a new exception to the prohibition on the use of force. The fourth section analyzes the relationship between humanitarian intervention and the world legal order and concludes that there are compelling moral, jurisprudential, and policy arguments which favor recognition of a doctrine of humanitarian intervention. Finally, the fifth section delineates the accepted criteria for a lawful intervention on humanitarian grounds. The article concludes that, at a minimum, international law accepts the unilateral or collective use of armed force to prevent mass slaughter of human beings and leaves open the question of whether humanitarian intervention might not be justified in other circumstances, for example, involving apartheid or the systematic denial of human rights other than the right to life.