Born Ugo Boncompagni, 1502. Bishop of Rome ("Pope") from 1572. Died 1585.
During his reign, Pope Gregory XIII sought to implement the decisions of the Council of Trent in the Catholic Church. He founded colleges and seminaries (including the Pontificia Universitas Gregoriana) to improve the general level of education of the priesthood. Throughout Europe, he supported the Society of Jesus (the jesuits) in their struggle against Protestantism.
Most famous of Pope Gregory XIII's works, however, is probably the fact that he was the instigator of the calendar reform that created the Gregorian Calendar, in 1582.