In 1998 he completed his habilitation in Freiburg in the fields of Medieval History and the historical sciences, with a focus on Florus of Lyon.
In the winter of 2003/4 he succeeded Tilman Struve as professor of History of the Early and High Middle Ages at the University of Cologne.
His research focused on the political, church and canonical history of the early and high Middle Ages.
He also studied intellectual and book history, especially of the Carolingian period, and specialized in the historical sciences, especially codicology.
His research, grounded in source and manuscript studies, led to revolutionary discoveries about the origin of the Pseudo-Isidorian Decretals, namely that they were assembled at the monastery of Corbie under the direction of Paschasius Radbertus in the later 830s.