Article 267 of the Treaty on the Functioning of the European Union (TFEU), and the preliminary ruling procedure it provides for, have a critical impact on the harmonious development of EU law and the way in which national and EU legal systems interact and communicate. It is considered ‘by far the most important aspect of the judicial system of the Community’. The procedure has provided a platform for the Court of Justice of the European Union (CJEU) to deliver seminal constitutional decisions that define the relationship between the EU and member states. On the other hand, it has been argued that the use of Article 267 has been overextended in a number of ways, for example, by pushing the boundaries of the type of bodies which can refer. This can lead to low quality rulings due to an overwhelming of the Court.
The main purpose of the preliminary ruling procedure is ‘to prevent the occurrence within the Community of divergences in judicial decisions on questions of Community law’, ensuring uniform interpretation of EU legal provisions across the 27 member states. As more than 50 per cent of the CJEU’s workload, it has brought a host of issues which the CJEU may not have had the opportunity to consider otherwise. It has allowed the court to develop fundamental principles of EU law, including direct effect, supremacy, indirect effect (i.e. the interpretation of national law in line with directives) and member state liability for breach of EU law.
One of CJEU’s key judgments was handed down in Case 26/62 Van Gend en Loos v Nederlandse Administratie der Belastingen  ECR 00001, which was the result of a preliminary ruling. It establishes the doctrine of direct effect: provided certain criteria are satisfied, an EU provision may give rise to a right which is enforceable by individuals in the national courts. The judges reasoned that if the intention of the founding member states was that individuals would not be able to invoke the Treaty before national courts, Article 267 TFEU would be meaningless. Although there is a close link between direct effect and supremacy, the latter was not affirmed by the CJEU until Case 6/64 Flaminio Costa v ENEL  ECR 585, also the result of a preliminary ruling. Presented as a challenge to the Court’s jurisdiction, the case encouraged it to formulate the supremacy principle: due to its special and original nature, Union law cannot be overridden by any domestic provision, however framed. The Court’s most convincing argument was the effet utile: if member states accept legal duties at international level, they should be prepared to allow the translation of those duties into daily practice, by means of judicial and other elements. Moreover, the principle of supremacy is essential to the uniformity of the EU legal edifice. The justifications of the fundamental principles of direct effect and supremacy are therefore close to the purpose of Article 267 and there is clear mutual dependence between preliminary rulings and the two principles.
However, the CJEU’s broad interpretation of how Article 267 itself can be used often leads to an overwhelming of the Court, increasing the time for the Court to answer the preliminary ruling to over 24 months in some cases. The type of bodies which can refer are a factor largely within the CJEU’s control. Although the Court has consistently refused to accept references from arbitration tribunals or administrative authorities not determining legal disputes, it has taken a broad, purposive approach of what constitutes a court or tribunal. The body referring a case must have a statutory origin and it must be permanent. It must also include an inter-partes procedure, have compulsory jurisdiction and it must apply the rule of law. These criteria were affirmed in Case C-17/00 De Coster v Collège des Bourgmestre et Échevins de Watermael-Boitsfort  ECR I-9445.
Under Article 267(3), there is an obligation to refer a case concerning a question of EU law by any national court or tribunal against whose decision there is no judicial remedy. The CJEU favours the concrete theory, meaning that Article 267(3) would apply to the highest court in a particular case. An example is in the case of Costa v ENEL, where the defendant had no right to appeal further at national level because the sum of money in dispute was too small. A preliminary ruling was therefore made as the national court had no further judicial remedy for the defendant
Nevertheless, despite the wording of Article 267(3), in Case 283/81 CILFIT v Ministry of Health  ECR 3415, the CJEU held that when a provision is so clear as to leave no scope for reasonable doubt, the last instance national court or tribunal has a discretion as any other national court or tribunal to refer a case. Known as the doctrine of acte clair, it has amended the Treaty obligation on courts of final instance to refer. In a large number of cases it is difficult to draw the line between the functions of the CJEU and national courts, where the answer to the preliminary ruling effectively determines the issue, leaving no role for the national court.
In Case 32/75 Fiorini (née Cristini) v Société Nationale des Chemins de Fer Francais  ECR 1085, where the question before the Court was whether a discounted rail ticket amounted to a social or tax advantage, it was held that the discount did amount to such an advantage, despite the CJEU stating that it could not decide the case itself. Therefore, the national court could only acknowledge the judgment of the CJEU and enforce it, rather than actually apply the interpretation of the law. As the Advocate-General in Costa v ENEL stated, there is a fine line between interpretation and application and the CJEU must be careful to stay on the side of interpretation.
In conclusion, the preliminary ruling procedure is essential to the uniformity of the Union, allowing the CJEU to develop and clarify key principles constructing the legal system of the European Union. Nevertheless, broad interpretation of what constitutes a court and tribunal and the increasing number of discretionary and mandatory references further expands the scope of the procedure. The continuous growth of referrals under Article 267 leads to an overwhelming of the Court. According to JHH Weiler, this may lead to the risk of lower quality rulings and the dilution of the normative effect of the rulings when the Court deals more with details than overseeing the development of Union law in a smaller number of important cases.
 Francis Geoffrey Jacobs and Andrew Durand (Butterworth Publishers Ltd, 1975) References to the European Court, pg. iii. Quoted by Anderson and Demetriou, pg. 24.
 Andreas Norberg (2005) ‘Preliminary rulings and the co-operation between national and European Courts’ (2006) University of Lund, 16.
 J.H.H. Weiler, ‘The European Court, National Courts and References for Preliminary Rulings — The Paradox of Success: A Revisionist View of Article 177 EEC’, in Henry G. Schermers, C.W.A. Timmermans, and A.E. Kellerman (eds), ARTICLE 177 EEC: EXPERIENCES AND PROBLEMS (Amsterdam: North-Holland, 1987) 368-369.