De Zwaan started his professional career in 1973 as a member of the The Hague Bar. From 1979 until 1998 he worked for the Dutch Ministry of Foreign Affairs in The Hague (European Integration Department and Legal Service) as well as in Brussels (Permanent Representation of the Netherlands at the EU). During his work in The Hague (1979-1983 and 1988-1995) he acted inter alia as Agent for the Netherlands Government in numerous cases before the Court of Justice of the European Communities in Luxembourg. As Legal Advisor of the Permanent Representation (1983-1988 and 1995-1998) he was involved in the negotiations on and the drafting of several European treaties, such as the Treaties of Accession of Spain and Portugal to the European Communities, the European Single Act and the Treaty of Amsterdam. In the period 1995-1998 he also was involved in the development of the Justice and Home Affairs cooperation of the European Union. In 1998 Jaap de Zwaan was appointed full time professor of the Law of the European Union at Erasmus University Rotterdam. In that capacity he was involved in teaching and research with regard to European law and policy. He also participated in several international frameworks of interuniversity cooperation. In the period 2001-2004 he served the Rotterdam Law School as Dean and in the period 1999-2001 he was also Dean of International Affairs of his faculty. From 2001-2004 he was also Dean of his Faculty. After being appointed as Director of the Clingendael Institute, he continues his work for Erasmus University Rotterdam on a part-time basis.