This chapter discusses how customary international law enters the sphere of EU law and to what extent it determines the relationship between the EU institutions, its member states and individual persons. Against this background, it is examined how customary international law is integrated into the EU legal order, what status it enjoys within that legal order, how it can be applied by EU institutions and whether it can serve as a benchmark for judicial review. Assuming that EU law is an autonomous domestic legal system, these questions are discussed from the perspective of EU constitutional law. Here it is argued that the Court of Justice of the European Union, in evaluating these issues, focusses too much on the idea of the autonomy of EU law. In order to provide provisions of customary international law with practical effect the validity of EU law must also be able to be reviewed on the basis of international law standards.