To a considerable degree EU law has been shaped by the European Court of Justice (ECJ). The Court has rightly been called a ‘motor of integration’ because, in the years of Euroscepticism and political standstill on the question of ‘deepening’ integration, it has largely created the ‘acquis communautaire’. Its case law, in particular in the field of the four freedoms, has made a significant contribution to the harmonisation and mutual recognition of national standards which, in turn, was essential for the creation of a true internal market.
The Court was able to attain this crucial role as a ‘quasi-law-maker’ because of its broad jurisdictional powers. In fact, the ECJ acts as ‘constitutional court’ of the EU exercising ‘judicial review’ over both the EU and its Member States. According to the ECJ’s own reasoning in Case 294/83 Les Verts v. Parliament  ECR 1356, the Community is:
“based on the rule of law, inasmuch as neither its Member States nor its institutions can avoid a review of whether measures adopted by them are in conformity with the basic constitutional charter, the Treaty. The Treaty established the Court as the judicial body responsible for ensuring that both the Member States and the Community institutions comply with the law.”
The ECJ, composed of judges from all Member States, exercises this judicial control mainly through ‘annulment actions’ directed against acts of the EU institutions, ‘infringement actions’ directed against Member States and ‘preliminary rulings’ providing guidance to national courts for the interpretation of EU law.