This essay examines the implications of new historical research on the origins of EU law for legal theory. Based on a review of the recent work of Morten Rasmussen, Bill Davies, Anne Boerger-de Smedt, Karin van Leeuwen, and Alexandre Bernier, the essay demonstrates how this historical research improves our understanding of two important themes in comparative law — comparative legal traditions and legal transplants. By examining the legal actors in different jurisdictions responsible for building an area of public law — the economic law of the fledgling European Communities — the new historical research contributes to the legal traditions literature on legal elites, which has traditionally focused entirely on private law. Historical research on the founding treaties and early litigation before the Court of Justice also contributes to the comparative law theory of transplants by illustrating how and why domestic law was transplanted into the relatively understudied context of supranational law. In this essay, however, I take issue with the claim that the recent historical research undermines the conceptualization of the European legal order, today, as a constitutional order. If constitutionalization is defined as the routine application and enforcement of EU law, then there is considerable evidence that the current legal order approximates more closely a federal legal system than public international law. Moreover, from a theoretical perspective, historical research demonstrating that the origins of EU law were more contested and contingent than originally believed does not in itself provide sufficient grounds for inferring that the current status of the European legal order is uncertain. As shown by a large body of social science research on path dependence, many resilient institutional systems have been put on self-reinforcing paths by contingent events and highly contested beginnings.