The last pope to resign, Gregory XII, did so in 1415, 10 years into his tenure, in the midst of a leadership crisis in the church known as the Great Western Schism. Three rival popes had been selected by separate factions of the church, and a group of bishops called the Council of Constance were trying to heal the schism. In an interview with Vatican Radio, Donald S. Prudlo, a papal historian at Jacksonville State University in Jacksonville, Ala., said that Gregory XII offered to resign so that the council could choose a new pope that all factions would recognize. It took two years after Gregory XIIís departure to elect his successor, Martin V.
Other popes known to have resigned:
Pope Celestine V: A recluse who only reluctantly accepted his election in 1294, Celestine V resigned and fled the Vatican after just three months to wander in the mountains. According to a history timeline on Christianity.com, the bishop who became his successor, Boniface VIII, was intent on ensuring that Celestine V did not become an example for future popes, and ordered Celestine V seized and imprisoned as he was about to sail to Greece. He died in custody in 1296 at the age of 81, and was declared a saint in 1313.
Benedict IX: One of the youngest popes, he was elected at the age of 23 in 1035, and became notorious for licentious behavior and for selling the papacy to his godfather, Gregory VI, and then twice reclaiming the position; he finally resigned for good in 1045, at the age of 33.
Gregory VI: Considered a man of great reputation, Gregory VI had thought Benedict IX unworthy of the papacy, and essentially bribed him to resign. He was recognized as pope in Benedictís stead, but when Benedictís attempt at marriage failed and he wanted to return to the papacy, a power struggle ensued. A council of bishops called upon Gregory VI to resign after less than two years in office because he had obtained the papacy through bribery.
By contrast, the resignation of Benedict XVI after an eight-year tenure will essentially be a retirement at the age of 85, after the pope showed increasingly public signs of fatigue in recent months. His last day as pope will be Feb. 28, coincidentally the feast day of a revered fifth-century pope, Saint Hilarius.