This chapter discusses the Court’s 1964 judgment, Dairy Products, where the Court declared that self-help mechanisms of reciprocity and retaliation, so important in many trade treaty systems, were comprehensively prohibited within the European legal order. This judgment is often rightly understood as marking an essential difference between European law and general international law. This chapter demonstrates that the principle announced in Dairy Products was also directly connected to the direct effect and supremacy doctrines of European law, again as shown by national court enforcement of treaty obligations in other treaty systems, and by the writings of judge Lecourt.