In January 1995 Rehberg joined the German Historical Institute of Rome (DHI), first with a post-doc fellowship, and then as a permanent member with particular expertise in the sources of the Roman Commune during the period 1400 to 1527 and in the Repertorium Germanicum project. His main works are centered around the so-called "prophecies of the popes" (1991), the social and political aspects relating to the higher Roman urban clergy and the papal confessors (poenitentiarii) (1999/2003), as well as the impact of the Colonna family on Rome (1998/99). In the 2001 summer semester he gave a lecture at the Faculty of the Conservation of Cultural Heritage of the University of the Tuscia in Viterbo. His study on the most ancient communal decrees of Rome, from 1515 to 1526, was translated into Italian in 2010. More recent works regarded the history of the S. Spirito hospital in Sassia and its religious order, the circumstances of the outbreak of the Great Schism of 1378, monastic immigration from beyond the Alps to the monasteries and convents of Rome and its hinterland and Roman heraldry. In June 2015 – in cooperation with the Facoltà Valdese (Rome) –  Rehberg organised the international conference "Indulgency Campaigns in the late Middle Ages, Martin Luther and the debate on indulgencies in 1517" (published in 2017 with De Gruyter).

On 28 May 2011, in Palazzo Borghese in Rome, he was awarded the Daria Borghese Prize for his merits in the study of the history of Rome. Since 2017 Rehberg has been the editor of the series "Ricerche dell'Istituto Storico Germanico di Roma".

Current Research Projects
Collection of sources on the City of Rome in the late Middle Ages and the Renaissance
Genealogy and Heraldry in the context of Federico Cesi "Linceo" (1585–1630): An example of the reception of the Middle Ages around 1600
The Holy Spirit Hospital in Rome as the centre of a hospital system of European importance

Memberships
- member of the "Società Romana di Storia Patria" (2002, effective member 2005)
- member of the association "Roma nel Rinascimento" (2004)
- member of the "Gruppo dei Romanisti" (2004)
- member of the "Comitato per la pubblicazione delle fonti relative alla terraferma veneta" (1997)

Main research interests
Social and ecclesiastical history of Rome in the late medieval period; sources regarding the history of the commune of Rome between the late medieval period and the Renaissance; aspects of the history of the religious orders (the Order of the Holy Spirit; alms collection; the multiethnic composition of the monasteries and convents of Rome); Roman heraldry