His main interests are papal history, the history of Italy and diplomatics. He has done his PhD-Thesis on papal charters in the period 896-1046 and an habilitation on the chapter of Saint Peter in Rome (11th-13th century), a prosopographical and institutional study. Together with Harald Müller (Aachen) he initiated the network on the formation of the papal church from the middle of the 11th century to pope Innocent III. He is a member of the Commission international de diplomatique
Johrendt – Geschichte – BERGISCHE UNIVERSITÄT WUPPERTAL (uni-wuppertal.de)