The European Court of Justice (ECJ) is a very powerful court compared to other international courts and even national courts of last resort. Observers almost unanimously agree that it is the preliminary references procedure that made the ECJ the powerful court it is today. In this article, we analyze the factors that lead national courts to use the procedure. We add to previous studies by constructing a comprehensive panel dataset (1982–2008) and identify the economic structure, familiarity with EU law, and tenure of democracy as new determinants.