This chapter examines the process whereby the concept of jus cogens was introduced into international law during the 1968–69 Vienna Conference on the Law of Treaties. The 1969 Vienna Convention on the Law of Treaties, the conference’s final product, declared that some rules of international law command universal authority, with Article 53 recognizing ‘peremptory norms of general international law’ (without specifying which norms counted as such). Yet the negotiations through which jus cogens entered into the law of treaties were marked by wide-ranging debates about the nature and limits of the treaty-making power, and ultimately about the basic structure and orientation of international law more generally. On the one hand were lawyers and diplomats from socialist and nonaligned states for whom the concept was potentially useful as a means of undercutting the legality of unequal treaties, colonial concession agreements, and other substantively unjust instruments. On the other hand were lawyers and diplomats from industrialized countries who were committed to the traditional principle of pacta sunt servanda—the ‘sanctity of compacts’—and deeply skeptical of any attempt to introduce a normative spectrum in which a select group of rules would have controlling authority over all others.