1962-66: Studies of law in Tübingen, Freiburg and Heidelberg.
1966: First State Examination in Law, University of Heidelberg.
1966-70: Postgraduate Judicial Service Traineeship (Referendariat) in the Higher Regional Court District of Karlsruhe; Research Assistant at the Max Planck Institute for Comparative Public Law and International Law.
1969: Masters of Laws (LL.M.) at McGill University, Montreal, Canada.
1970: Second State Examination in Law, Stuttgart.
1971-74: Research Assistant at the University of Heidelberg (Professor Dr. K. Doehring).
1972: Dr. jur. (Ph.D./doctorate in Law), supervisor: Professor Dr. K. Doehring, University of Heidelberg.
From 1974: Research Fellow at the Max Planck Institute for Comparative Public Law and International Law.
1977: Habilitation (Dr. jur. habil.) in Public and International Law at the University of Heidelberg.
1977: Assistant of the Vice-President of the Federal Constitutional Court (Prof. Dr. W. Zeidler) and Senior Legal Officer at the Ministry of Justice, Rheinland Pfalz.
1979: Appointment to the chair for Public Law, International Law and European Law at the University of Konstanz.
1979-91: Calls to chairs from the Universities of Speyer, St. Gallen, Berlin, Kiel and offer for the position as Institutional Director of Public International Law at Humboldt University, Berlin.
Since 2000 Member of the Advisory Board of the German Federal Office for Migration and Refugees (Bundesamt für Migration und Flüchtlinge, BAMF).
2003: Bundesverdienstkreuz 1.Klasse (Order of merit of the Federal Republic of Germany).
From 12/2007: Acting Chair of the Federal Advisory Council for Migration and Refugees.
Since 2010 Professor emeritus at the University of Konstanz and Director of the Research Center for Alien and Asylum Law.
1988-92: Judge at the Administrative Court of Justice of Baden-Württemberg.
Since 1994: Head of the Research Centre "European and International Immigration and Asylum Law”.
Since 1994: Co-editor of the "Baden-Württembergische Verwaltungsblätter" (Baden-Württemberg Administrative Law Reports); co-editor of the "Zeitschrift für Ausländerrecht und Ausländerpolitik" (Aliens Law and Policy Journal).
1998-2002: Chairman of the International Law Association Committee on Refugee Procedures.
1999-2006: Member of the German Advisory Council on Science (Wissenschaftsrat).
2000-2001: Member of the Governmental Commission on Immigration Reform (Zuwanderungskommission).
2007: Honorary Doctorate at the University of St. Gallen.