The prohibition of the use or threat of force is a fundamental principle and peremptory norm of public international law. The prohibition assumed its prevailing form and content only after the Second World War, but the underlying demand for peace and security is as old as humanity itself. The course offers a historical overview of theorectial conceptions and legal and other means that have been developed with a view to justify, restrict or outlaw the resort to war or armed force in international relations. It also provides a detailed introduction into the contemporary international regulation and practice of the prohibition of the use or threat of force as well as into various efforts that seek to legitimize or justify unilateral resort to force.