This paper analyses the phenomenon of deference to Member State courts from an empirical perspective, focusing on the field of free movement law. Drawing on an original data set of ECJ rulings on the four freedoms, it looks into how frequently the Court of Justice delegates decisions to its national counterparts, and which ones. The results reveal that the role of Member State judges has grown both quantitively and qualitatively over time. The paper investigates potential reasons for this development, which range from the maturing of the European legal order, to increases in the ECJ’s workload, to improvements in compliance at the domestic level. It also addresses the consequences for the unity of EU law and the idea of cooperation which underlies the preliminary references system.