The main mission of the papal nunciature at the imperial court in Prague at the turn of the seventeenth century was the diplomatic representation of the Holy See, the coordination of the Counter‑ Reformation in a religiously mixed area and the supervision of church reform in alignment with the instructions of the Council of Trent. The papal nuncios could have also had various unofficial commitments to meet in favour of some of their relatives or other individuals. This is very well documented in Giovanni Stefano Ferrari’s private registry. He held the office of papal nuncio at the imperial court between 1604‒1607. The private registry contains a comprehensive collection of letters with members of the College of Cardinals. An analysis of this correspondence demonstrates that the nuncio utilised his diplomatic status in carrying out various tasks and commitments in favour of the cardinals, who – like he himself – belonged to the clients of the Aldobrandini family; they were also at times his own friends or clients.