Before turning into its present form, this book has had many lives that span over almost a decade. Its origins can be traced back to many heated discussions over Europe's polity and transnational fields within a small group of friends (Antonin Cohen, Mikael Madsen, Guillaume Sacriste) and, later, within a more formal research network named Polilexes (‘Politics of Legal Expertise in European Societies’) and financed by the French Agence nationale de la recherche. Throughout its development, the critical input that I received from Yves Dezalay and his tireless passion for transnational research have been widely inspirational. All these exchanges grew into an individual project parallel to the collective and collaborative one, on which I started working during my stay at the European University Institute in Florence as a Marie Curie fellow. There, I greatly benefited from the critical mass of EU scholarship and the interdisciplinary atmosphere that is so particular to that place. Many discussions and debates with wonderful scholars such as Bruno de Witte, Yves Mény, Christian Joerges, Karen Alter, Kiran Patel and Heike Schweitzer have helped me a lot. Eventually, the project was turned into an Habilitation à diriger des recherches that I presented at the Université Paris 1-Sorbonne in March 2010, with the support of Bastien François. Along the way, some early parts of this overall research were published in a variety of disciplinary fields including law (the European Law Journal and Law and Social Inquiry), sociology (the American Journal of Sociology and International Political Sociology) and political science (European Political Science Research, Revue française de science politique, etc.), and I am therefore indebted to my co-authors and co-editors (Bruno de Witte, Antonin Cohen, Didier Georgakakis, Mikael Madsen, Stephanie Mudge and Cécile Robert) as much as to various referees for pushing me forward. I turned most of the Habilitation into a book entitled L'Union par le droit. L'invention d'un programme institutionnel pour l'Europe published in 2013 in a series co-directed by Patrick Le Gales at the Presses de Sciences Po. As the present book was progressively taking shape, I had countless opportunities to present sections of the manuscript in many venues. In particular, I benefited from early book talks at Sciences Po-Lille, Lyon, Strasbourg and Paris, the American Bar Foundation in Chicago, the London School of Economics, Siena University, the Central European University, and Berlin's Wissenschaft Zentrum. It was, however, during the academic year 2013–14 that I completed the English-language version of this manuscript (Meg Morley was the main translator of the text, and I wish to thank her here), as I benefited from invitations from Columbia University Law School (as an Alliance visiting professor during the Fall semester) and from New York University Law School (as a Senior Emile Noël fellow during the spring semester). That year spent in New York proved instrumental to my completing this project, and again I benefited immensely from feedback after talks given at New York University, Columbia, Cornell, Princeton and the American University in Washington, and discussions with scholars and experts of the EU such as Grainne de Búrca, Daniel Kelemen and Peter Lindseth. Of course, all these years spent in the academic trenches have a special fil rouge, Stéphanie Hennette-Vauchez, with whom I have loved to share this intellectual and personal journey.