This chapter discusses the Court’s 1964 judgment, Costa v. ENEL, where the Court declared the supremacy of European law, thus requiring national courts to resolve conflicts between European law and national law in favour of the domestic application of European law. The supremacy of European law is often understood as the partner of the direct-effect doctrine and the result of the Court’s ‘functionalist’ approach to the development of the European legal order. This chapter demonstrates that the supremacy doctrine of European law was also motivated by the Treaty of Rome’s prohibition on the unilateral adoption of safeguard measures by the member states, as shown both by the text of the Court’s judgment and by the writings of judge Lecourt.