This editorial explores how two developments—the humanization and fragmentation of international law—permeate all aspects of jus cogens: its foundations, content and consequences. The authors are particularly intrigued by the question of how the unceasing popularity of jus cogens can be reconciled with its limited role in legal practice. It has often been observed that jus cogens owes its proliferation to the increased focus on human rights. This, in turn, has yielded two effects. First, such focus on human rights has triggered greater attention for the enforcement of peremptory norms. Secondly, it has put the responsibility of non-state actors for violation of jus cogens norms on the agenda. It may not be too far-fetched to understand the reticence of states to accept the expansion of jus cogens and its effects against the background of the fear that this will weaken the power of the state, whereas one might argue that the state is rather in need of reinforcement, in view of the manifold challenges it is confronted with. Next to the process of ‘humanization’ of international law, the appeal of jus cogens can be explained from the international lawyer’s desire for a single and coherent system of law, including a more clearly established hierarchy of norms. This aspiration is primarily infused by the concern for ‘fragmentation’ of international law. However, as in the case of humanization, countervailing factors prevent a further expansion of jus cogens in international law. For one thing, jus cogens, belonging to the realm of general international law, is too coarse and inflexible to be of effective use in special sub-fields of international law. A second explanation for the limited role played by jus cogens is that specialized international or regional courts and tribunals are hesitant or may even lack the competence to pronounce on a conflict between their legal order and other branches of international law.