This Working Paper explores how the entanglement between Law and European polity-building was initially established. To this aim, it follows the short historical sequence in which EC institutions and policies set up by the Rome Treaties were first invented and formalized. It considers the early emergence of transnational microcosms of practitioners of European politics, judiciary, bureaucracy and market in Brussels, Strasbourg and Luxembourg. Often endowed with legal credentials and well-connected to legal scholarship and judiciaries, these first office holders shaped the foundational concepts and theories through which EC-specific institutions and policies soon established themselves. The paper therefore contends that lawyers and their ad hoc legal theories were integral to the transformation of the institutional and policy complex set up by the Paris and the Rome Treaties (three separate Communities, a complex set of institutions, a variety of policies) into one ‘constitutional settlement’ providing a unitary understanding of this emerging transnational institutional terrain.