Pope Benedict XVI's resignation is an extremely rare event in the two millennia history of the Catholic Church.
Marcellinus: This early church pope abdicated or was deposed in 304 after complying with the Roman emperor's order to offer sacrifice to the pagan gods.
Benedict IX: Sold the papacy to his godfather Gregory VI and resigned in 1045.
Celestine V: Overwhelmed by the demands of the office, this hermetic pontiff stepped down after five months as pope in 1294. Pope Benedict XVI prayed at his tomb in the central Italian city of L'Aquila in 2009.
Gregory XII: The last pope to resign, Gregory XII stepped down in 1415 to help end a church schism.