AbstractThis introductory Article sets out the background and summarises the content of the Symposium on ‘The Court of Justice of the European Union (CJEU) as a Relational Actor’. Observing that the CJEU has been a key player in shaping European legal integration, the Symposium takes as its starting point that courts – and the CJEU is no exception in this regard – are unable to drive developments in isolation. For the Court to carry out its role as guarantor of the Treaties and guardian of the rule of law in the Union, it needs not to isolate itself but to interact with other European Union (EU) and national institutions. Relations, the Symposium argues, are not only unavoidable but also legitimate and even vital for the adequate execution of the judicial function. This introduction briefly explores six of the Court’s most important relationships: those with the other EU institutions; the courts of the Member States; the Member States themselves; the parties appearing before it; other international courts; and the general public. It then summarises the contributions and highlights how they complement each other in examining the CJEU as a relational actor.