The preliminary reference procedure under Article 267 of the Treaty on the Functioning of the European Union (TFEU) is instrumental for the so-called “judicial dialogue” within the European Union (EU). The goals of the preliminary reference procedure are to ensure the uniform interpretation and application of EU law and to contribute to the harmonious development of the law throughout the EU. It was through the preliminary reference procedure to the Court of Justice of the European Union (CJEU) that the principles of direct effect and supremacy were developed. It took many years before the first request by a Constitutional Court was sent to the CJEU. So far, the Constitutional Courts of Belgium, Austria, Lithuania, Italy, Spain, France, Germany, and most recently Slovenia, have sent requests for preliminary rulings to the CJEU. By far the most active of these in sending requests has been the Belgian Court. The Portuguese Constitutional Court has indicated that it can request preliminary rulings from the CJEU but is yet to do so. In the other Member States (MS) with Constitutional Courts, references have not been sent yet, although worthy occasions in terms of EU-law-related cases have occurred, as also observed in various contributions in this special issue. These MSs include Bulgaria.