This paper examines two aspects of the interplay between the ECJ and the national courts. It first looks at the degree of specificity of the rulings delivered by the ECJ in preliminary references. It distinguishes three categories of cases depending on the degree of specificity (outcome, guidance and deference) and seeks to explore the rationale and function of each. It then looks at the role of national courts in building the edifice of EU law and the varying ways in which they perceive the preliminary reference procedure. It concludes that the process towards the constitutionalisation of the EU Treaties could not have advanced without the cooperation of the national courts. While such cooperation has taken a variety of forms ranging from encouragement to acquiescence, national courts have essentially played a very constructive role in legitimating integration.