To the great joy of some, and even greater chagrin of others, the margin of appreciation has become a cornerstone of international rights adjudication. Within less than 40 years, the doctrine has made it from Strasbourg into courtrooms across the world. This thesis studies the use of the margin of appreciation by the Court of Justice of the European Union. By the same token, it studies the Court and the EU as such, and the remarkable evolution both have undergone during the past decades. The research focuses on the Court’s jurisprudence on free movement rights and Member State restrictions thereof. After conceptually defining the margin of appreciation, the thesis investigates the law and practice of the doctrine. The analysis is based on an empirical survey of free movement case-law, which covers around 250 judgments from 1974 until 2013. The data expose some fundamental changes in the review behaviour of the Luxembourg Court since the 1970s. Its jurisprudence is evermore marked by self-restraint and decentralisation, a development which manifests itself in two ways. For one thing, the Court increasingly grants national legislatures the freedom to make particular policy decisions. For another, it passes more and more review duties onto national courts. The thesis discusses the implications of these phenomena for review tools such as proportionality analysis. Likewise, it provides a normative assessment of the Court’s practice. It is shown that the changes that have occurred in free movement law have to do with some broader changes in the European project. Over the past two decades, the EU has embraced a series of constitutional goals which take it far beyond its original mission statement. These goals suggest that it is, at times, desirable that the European judges renounce control over Member State acts and, thus, practise passive virtues.