From 1875 he was a member of the Monumenta Germaniae Historica, of which, he subsequently became head of its diplomata section. In this capacity, he performed meticulous and expert edition of medieval documents, including over 1300 royal documents of the 10th century (Diplomata regum et imperatorum Germaniae).
In 1881 he established the Österreichisches Historisches Institut (Austrian Historical Institute) in Rome, serving as its director until 1901. From 1897 to 1908, he was president of the Bavarian Academy of Sciences.
Selected works
Beiträge zur Diplomatik (8 volumes, 1861–1882).
Acta regum et imperatorum Karolinorum digesta et ennarata. Volume 1: Urkundenlehre (1867), Volume 2: Urkundenregesten.