The EEC Treaty is first and foremost an agreement governed by international law. The legal regime that it creates, however, is unique among international agreements in a number of respects-notably the way that its rules penetrate the legal systems of its members. In the famous Van Gend en Loos case1 the European Court of Justice (ECJ) declared that it created a ‘new legal order’.2 Nevertheless, as a treaty it derives its binding force from international law, and many of the constitutional problems which Member States have encountered in implementing its obligations have arisen from traditional thinking about the status of international treaties in national legal systems.