He was Professor of European Law at University College London (1993-2004) and Vice-Dean of the Faculty of Laws. He was Professor of European Law and head of the Department of Law, University of Durham 1990-1993, and a visiting professor at numerous Universities including University College Dublin (Ireland), the University of Siena and the College of Europe in Bruges and Warsaw.
He is best known for his large volume of published work on the law concerning migrant workers in the EU and European Social Law. He was a member of the High Level Panel on the Free Movement of Workers established by the European Commission, chaired by Simone Veil. He was Vice-Chairman of the panel chaired by Lord Templeman culminating in a Report on immigration in the UK.
He was also an academic organizer and administrator, establishing permanent centers for research and teaching on European Union law: the Durham European Law Institute (University of Durham, U.K.) and the Centre for the Law of the European Union (University College London).
He was the coordinator of numerous major international research projects and international conferences. He co-founded the Irish Forum for European Law.
He has written or edited over 200 academic publications specializing in EU law.
He was also a co-founding editor and joint editor of the European Foreign Affairs Review. He was a member of the editorial, advisory or scientific boards of a large number of journals specializing in EU Law including many years as as an editor of the Common Market Law reciew. He was Editor-in-Chief of the European Monograph book series.
Amongst the works with which he is most associated are Legal Issues of the Maastricht Treaty (Chancery Law Publishing, 1994) and Legal Issues of the Amsterdam Treaty (Hart, 1999).
He gave guest lectures in Universities all over Europe. He gave evidence to committees of the European Parliament and the House of Lords on EU matters, and advised the Netherlands Government on certain aspects of the Maastricht Treaty. He was also an advisor to the European Ombudsman.
In later years, he practised law and pleaded before the European Court of First Instance and the European Court of Justice .