Each year the European Court of Justice delivers over a thousand decisions on the basis of EU law that affect the Member States as well as the lives of their citizens. Most of these decisions are the result of requests for a preliminary ruling sent by national courts and tribunals seeking an interpretation of EU law. While this procedure is seen as central to the transformation of Europe, significant ambiguity remains on why it is used, and who is primarily responsible for its success. The current book examines the practice of the preliminary reference procedure. By approaching it from the perspective of those who participate in it, the study takes on prevalent assumptions about the how and why of national court cases that reach the European Court of Justice through a request for a preliminary ruling. This empirical research will appeal to scholars engaged in the relationship between law and European integration as well as practitioners and litigants interested in the practice of the preliminary reference procedure.