In contemporary Europe, the Court of Justice of the European Union functions as a uniquely authoritative international court. Its key doctrines—direct effect and supremacy—ensure a relatively effective enforcement of European legislation compared to standard international organizations. Likewise, the system of preliminary references sent from national courts to the ECJ has given voice to private litigants across Europe to pursue the rights given them by the European treaties and legislation. In fact, the ECJ has today become so central in the EU that sympathetic academic observers claim it has become a European Supreme Court of sorts and that it has built a constitutional, proto-federal legal order. How did this happen? How could a set of international treaties—the Treaties of Rome - gradually attain the status of a constitution or at least lead to what might be termed a “constitutional practice”? This has been the key question of a new emerging field of legal history focusing on the foundation and development of European public law. Drawing on new evidence from available private, state, and European archives, the new historical research goes behind the scenes to unveil the world in which European public law was created. The result is a more complex but also deeper understanding of the social, institutional, legal, and ideological roots of European public law. This panel will bring together the latest results from this new field thereby offering a solid basis for reflection about the role of law in the European construction past and present.