The History of the Popes (Vol. XVI) : From the Close of the Middle Ages focuses on the pontificate of Pope Pius IV (1559–1565). The book details the conclusion of the Council of Trent specifically the formidable challenges Pope Pius IV faced in bringing the Council to a close and implementing its decrees, which were crucial to the Counter-Reformation, as well as the Catholic Church's political maneuvering in the face of the Reformation. Other aspects of Pope Pius IV’s reign are also examined in depth, such as how papal ambassadors and delegates navigated complex political relations with European princes particularly the Spanish Empire alongside authentic accounts of internal negotiations, the handling of doctrinal disputes, and the resolution of significant trials of the era. This comprehensive work spans a total of 40 volumes chronicling the history of 56 popes.