This volume brings together the correspondence between St. Charles Borromeo, archbishop of Milan, and the Barnabite Congregation and some of its members individually from 1568 to 1584, that is, from the time he joined the Ambrosian archdiocese to his death. The historical research conducted by the Curator, Prefect Emeritus of the Vatican Apostolic Archives, follows in the line of the very rich and ever-growing historiographical production related to the figure of Borromeo.