This chapter focuses on the Court of Justice of the European Union (CJEU), the judicial arm of the European Union. The CJEU consists of three courts: the Court of Justice, the European General Court, and the Civil Service Tribunal. Its mission is to ensure that ‘in the interpretation and application’ of the treaties of the Union ‘the law is observed’. The chapter first traces the history of the CJEU before discussing issues of structure and procedure, the extent of the Courts' jurisdiction, and their role in the promotion of European integration. It then considers the criticism directed at the CJEU for the way it exercises its judicial powers, and more specifically the reaction of member states to its ‘judicial activism’. It concludes with an assessment of the main challenges facing the EU courts.