Gregory XIII (1502-1585), pope from 1572 to 1585, who carried out the reform of the Julian calendar, producing the system currently in use throughout the Western world and in parts of Asia (the Gregorian calendar). Born Ugo Buoncompagni in Bologna, Italy, he was one of the prominent theologians at the Council of Trent and was created cardinal by Pius IV in 1564. On the death of Pius V, Gregory was elected pontiff.

Gregory issued a new edition of the compilation of canon law Corpus Iuris Canonici (1582), expended large sums for education and the building of colleges, and undertook many great public works, such as the papal palace—subsequently the residence of Italian kings and presidents—on the Quirinal Hill (Monte Quirinal) in Rome. He vigorously spread anti-Protestant propaganda, attempted to form a coalition against the Protestants, and aided Philip II, king of Spain, in an attack on the largely Protestant Netherlands. Gregory's many efforts to restore Roman Catholicism or Roman Catholic supremacy in Europe, although largely unsuccessful, had an effect not only in the southern portions of the Netherlands but also in Poland and Austria, as well as in Bavaria and other parts of Germany.