The world’s nations have developed an extensive body of rules and regulations governing their relationships with each other, and with the non-state actors that are also subject to their law. These rules, collectively known as international law, define the legal responsibilities of States in their interactions and govern such issues as the creation and dissolution of states, the use of force (international humanitarian law), the conduct of war (norms of armed conflict), human rights, refugees, crimes, the environment, labor, international trade, outer space and the treatment of diplomats.

The oldest sources of international law are treaties, conventions and covenants between States. Other important sources are customary law and jus cogens, which describe peremptory norms of international law from which no derogation is permitted by treaty.

While the subjects of international law were traditionally limited to state relations, today it includes not only matters involving states but also, in increasing frequency, international organizations and individual individuals. The value of international law is derived not only from its binding nature, but also from the clarity and sense of common purpose it imparts to the global community of States, international organizations and other international actors.

Although there is no single international police force or system of law enforcement, the International Court of Justice, UN-created ad hoc tribunals and other tribunals administered by the United Nations perform many key functions that would be impossible for States to undertake alone (such as investigating and prosecuting perpetrators of genocide, crimes against humanity, war crimes and violations of the laws of war). In addition, an increasingly broad range of international agreements—from free trade to agreements on the conservation of endangered species to rules on maritime safety and admiralty—make up the broader field of modern international law.