International law (IL) is in principle of a dispositive nature. The international community, which is the author of international law, is decentralized. Although there is no formal hierarchy within IL as a whole, there are some hierarchical elements. One of them is jus cogens, a legal category that can be found in the Vienna Convention on the Law of Treaties (VCLT), in different guidelines of the Internatioanl Law Commission (on fragmentation, reservations), as well as in the Articles on the responsibility of states and international organisations and in international and national jurisprudence. This article analyses jus cogens from two perspectives: normative/static (e.g. the role of Article 53 VCLT; jus cogens as a rule, and its distinguishing criteria: formal, sociological and axiological; its relation to erga omnes obligations; and the catalogue) and functional/dynamic (e.g. ius cogens and law-making, especially the conclusion and extinction of treaties; its impact on the application and interpretation of IL and normative conflicts in IL). The article concludes that: ius cogens belongs to contemporary IL, but sometimes expectations are too high; its significant distinctive criterion is an axiological one (peremptory norms protect the most important values for humans and states/peoples) – as a result jus congens can be changed, but its replacement by other rule is inconceivable without a fundamental change in the nature of international community; the legal basis is jus necessarium (the objective universal legal consciousness of states); jus cogens restricts the freedom to undertake international obligations (not only treaties); it counters the fragmentation of IL; it gains some control, through the law of responsibility, over factual acts of IL subjects and over domestic law; and jus cogens shows a tendency to extend its effects on the activities of non-state actors, including their criminal responsibility.