1982 – 1985 Student Assistant at the Chair for Civil Law and Modern History of Law of Professor Dr. Hermann Dilcher, Faculty of Law, Ruhr-Universität Bochum

01/1986 First State Exam in Law

1986 – 1988 Research Assistant at the Chair for Public Law III (Public International Law) of Professor Dr. Dr. h.c. mult. Knut Ipsen, Faculty of Law, Ruhr-Universität Bochum

1989 – 1997 Research Associate (until November 1989), then Acadamic Assistant at the Chair for Public Law III (Public International Law) of Professor Dr. Dr. h.c. mult. Knut Ipsen, Faculty of Law, Ruhr-Universität Bochum

04/1989 Second State Exam

12/1992 Doctor in Law, Faculty of Law, Ruhr-Universität Bochum, title of the doctoral thesis: "Grundgesetz und Kriegswaffenkontrolle - Erfüllung des Verfassungsauftrags durch den einfachen Gesetzgeber? Verfassungsanspruch und Rechtswirklichkeit"

07/1994 - 06/1996 Habilitation fellowship from the German Research Foundation

Winter Semester 1996/97 Professor ad interim of Public Law, Faculty of Law, University of Münster

12/1996 Habilitation, Venia Legendi for Public Law
Habilitation treatise: "Die Außenwirtschaftsfreiheit"; Habilitation presentation on the topic of: "Democratic Legitimation of the Judiciary of the European Communities"

1997 – 1999 University lecturer for Public Law, Faculty of Law, Ruhr-Universität Bochum

04/1997 – 07/1997 Professor ad interim for Public Law, Faculty of Law, University of Cologne

10/1997 – 02/1998 Professor ad interim for Public Law, Faculty of Law, University of Freiburg

04/1998 – 07/1998 Professor ad interim for Public Law, Faculty of Law, University of Regensburg

03/1999 – 11/2001 University Professor for Public Law, Faculty of Law. University of Münster

since 12/2001 University Professor for Public Law, Chair for Public International and European Law, Faculty of Law, Leibniz University Hannover

04/2004 – 03/2007 and 10/2008 – 03/2009 Dean of the Faculty of Law, Leibniz University of Hanover

04/2009 – 03/2011 Member of the Senate, Leibniz University Hannover and Member of the Senate, Niedersachsen Institutes of Technology

07/2012 Best Lecture Award

since 01/2015 President of Leibniz University of Hanover