A little more than four decades ago, the European Court of Justice declared that the law of the European Communities constitutes the supreme law of the Member States. The national institutions, most importantly the national courts, were to apply rules of Community law and, in so doing, were required to set aside conflicting provisions of national law, however framed. Since then, this judicially formulated constitutional principle has been developed and restated in later judgments by the ECJ. However, during the same period the absolute character of the principle has been continually challenged by the Member States.