Petersen is Professor of Public Law, International Law, and EU Law at the University of Münster since February 2015. Previously, he was a Research Fellow at the Max Planck Institute for Comparative Public Law and International Law in Heidelberg (2004-2006), a Visiting Doctoral Researcher (2006/07) and Hauser Research Scholar (2012/13) at the NYU School of Law, and a Senior Research Fellow at the Max Planck Institute for Research on Collective Goods in Bonn (2007-2015). His research focuses on international human rights and domestic fundamental rights, constitutional theory, empirical methods in constitutional law, and the sources of international law.