The article explores and compares the role of the judiciary in US constitutionalism and European integration. The development of these two legal orders has raised very similar constitutional problems and the purpose of the inquiry is to highlight the similar legal solutions devised by the US Supreme Court and the European Court of Justice. Four aspects have been singled out to show the importance of constitutional adjudication by courts in both experiences: the foundations of judicial review, supremacy of federal/Community law over state legislation, the protection of fundamental rights, and the dialogue among constitutional courts.