Peremptory norms are non-derogable standards of international public policy which impose limits on how far governments, politicians, and diplomats can further their own goals in making international transactions. For example, certain core norms prohibit aggressive war, safeguard self-determination, and protect basic human rights in both peace and wartime. This monograph analyses the questions raised by the legal effects of peremptory norms of international law (jus cogens) in the light of their increasing importance in determining the permissible limits on the action of State and non-State actors in multiple areas of international law.