This Living Reviews article evaluates the most important strains of social science research on the impact of the European Court of Justice (ECJ) on integration, EU-level policymaking, and national legal orders. Section 2 defines the concepts of judicialization and governance, and discusses how they are related. As the article demonstrates, the "constitutionalization of the EU," and its effect on EU governance, is one of the most complex and dramatic examples of judicialization in world history. Section 3 discusses the institutional determinants of judicial authority in the EU in light of delegation theory. The European Court, a Trustee of the Treaty system rather than a simple Agent of the Member States, operates in an unusually broad zone of discretion, a situation the Court has exploited in its efforts to enhance the effectiveness of EU law. Section 4 focuses on the extraordinary impact of the European Court of Justice, and of the legal system it manages, on the overall course of market and political integration. Section 5 provides an overview of the process through which the ECJ's case law-its jurisprudence-influences the decision-making of non-judicial EU organs and officials. Section 6 considers the role of the ECJ and the national courts in monitoring and enforcing Member State compliance with EU law, a task that has provoked a steady Europeanization of national law and policymaking.