Abstract: The dynamics by which norms emerge and spread in international society have been the subject of strikingly little study. This article focuses on norms that prohibit, both in international law and in the domestic criminal laws of states, the involvement of state and nonstate actors in activities such as piracy, slavery, counterfeiting, drug trafficking, the hijacking of aircraft, and the killing of endangered animal species. It analyzes the manner in which these norms have evolved into and been institutionalized by global prohibition regimes and argues that there are two principal inducements to the formation and promotion of such regimes. The first is the inadequacy of unilateral and bilateral law enforcement measures in the face of criminal activities that transcend national borders. The second is the role of moral and emotional factors related to neither political nor economic advantage but instead involving religious beliefs, humanitarian sentiments, fears, prejudices, paternalism, faith in universalism, the individual conscience, and the compulsion to proselytize. The ultimate success or failure of an international regime in effectively suppressing a particular activity depends, however, not only on the degree of commitment to its norms or the extent of resources devoted to carrying out its goals but also on the vulnerability of the activity to its enforcement measures.