If there is a conflict between a provision of national law and a directly effective provision of EC law, administrative authorities are obliged to set aside national law if consistent interpretation of the latter is not possible, and eventually to apply directly effective provisions of Community law instead. This obligation, which the European Court of Justice (ECJ) has clearly formulated for both primary EC law and secondary legislation, is hereinafter for reasons of convenience referred to as the ‘Costanzo obligation’. Although this obligation may seem entirely clear cut and explicable from a European perspective, problems may occur from the point of view of national constitutional and administrative law. In this paper, the effect of the ‘Costanzo obligation’ with regard to the principle of legality in Germany, France and the Netherlands is discussed.