Since the early 1990s, the European Union has included human rights conditionality clauses in its association agreements and other international trade and cooperation agreements. The purpose of these clauses is to entitle a party to take appropriate measures, including suspension of the agreement, in the event that the other party violates human rights or democratic principles. This book provides an account of the evolution of these clauses, their scope, and their operation, and analyses the EU's responsibility, under international law, to implement these clauses domestically. Based on this examination, it explores the extent to which the EU has the legislative competence to include such clauses in its international agreements, and concludes by considering the implications of ultra vires agreements in EU law. The study offers theoretical insights into aspects of international law as well as EU constitutional and external-relations law. Its practical conclusions have major implications not only for the application of human rights clauses, but also for the EU's international treaty practice more generally.