This book investigates the phenomenon of deference to Member State authorities in EU free movement law. It enquires into the decision-making latitude which the European Court of Justice grants national institutions by means of two deference doctrines: the margin of appreciation and decentralized judicial review. At the same time, it sheds light on a number of broader developments in European law. These include changes in the intensity of judicial review, the relationship between centre and periphery, the interaction between political and adjudicative processes, and the division of powers between EU and Member State courts. Drawing on an original data set of free movement cases from 1974 to 2013, the book examines how and which decisions the Court defers to national institutions. It tests the impact of twelve variables on the practice of judicial deference. The results reveal that free movement law and the internal market have substantially changed over the past four decades. The Court has scaled down its involvement in and reduced its control over Member State affairs. The book argues that these new-found ‘passive virtues’ are linked to the legal, political, and institutional changes that have taken place in the EU.