He was Pope from 1566-1572 and one of the foremost leaders of the Catholic Reformation. Born Antonio Ghislieri in Bosco, Italy, to a poor family, he labored as a shepherd until the age of fourteen and then joined the Dominicans, being ordained in 1528. Called Brother Michele, he studied at Bologna and Genoa, and then taught theology and philosophy for sixteen years before holding the posts of master of novices and prior for several Dominican houses.

While out of favour for a time under Pope Pius IV who disliked his reputation for excessive zeal, Ghislieri was unanimously elected a pope in succession to Pius on January 7, 1566. As pope, Pius saw his main objective as the continuation of the massive program of reform for the Church, in particular the full implementation of the decrees of the Council of Trent. He published the Roman Catechism, the revised Roman Breviary, and the Roman Missal.