Jus cogens, also known as peremptory norms, refers to a category of norms that govern customary international law. These norms are considered fundamental principles of international law that are accepted by the international community of states as norms from which no derogation is permitted. Jus cogens norms are mandatory and compel lawmakers to act in a specific manner. If a provision of a treaty conflicts with an existing jus cogens norm, that provision is invalid insofar as it violates jus cogens. For a norm to be considered a jus cogens norm, its mandatory effect must be universally accepted by the international community.

There is no universal agreement regarding precisely which norms are jus cogens nor how a norm reaches that status, but it is generally accepted that jus cogens bans genocide, maritime piracy, enslaving in general (i.e. slavery as well as slave trade), wars of aggression and territorial aggrandizement, torture, and refoulement. These norms are considered to be of such importance that states may not derogate from them via treaty or customary international law norms.

Jus cogens is now referred to in several legal instruments within and beyond the law of treaties. The 1969 and 1986 Vienna Conventions on the Law of Treaties stipulate that a treaty is void if it conflicts with jus cogens. According to the Articles on Responsibility finalized by the International Law Commission in 2001 and 2011, states as well as international organizations shall cooperate to bring to an end any serious breach of jus cogens, and shall not recognize as lawful a situation that has been created by such a breach.

There is no authoritative list of Jus Cogens, as there are no clear guidelines or criteria for identifying them. Some scholars would expand any list of Jus Cogens, and others would question its very existence. Nevertheless, the following list includes the least controversial examples, according to Brownlie, and acknowledged by the United Nations International Law Commission: prohibition of crimes against humanity, prohibition of genocide, prohibition of slavery and the slave trade, prohibition of torture, prohibition of piracy, and the principle of non-refoulement.