Seven years later appeared his great work, Regesta Pontificum Romanorum ab Condita Ecclesia ad Annum p. Ch.

This work made him well known, but he had still to earn a livelihood; he therefore again entered the university, this time as a student of medicine, at Berlin and later at Vienna.

from Berlin in 1853, he engaged in practise in that city for a year, and then became one of the editors of the Monumenta Germaniae Historica.

In 1862 Jaffé was appointed assistant professor of history at Humboldt University of Berlin, where he lectured on Latin paleography and Roman and medieval chronology.

Jaffé wrote, in addition to the above-mentioned works, Geschichte des Deutschen Reiches unter Lothar dem Sachsen, Berlin, 1843; Geschichte des Deutschen Reiches unter Konrad III.