Pope Benedict XVI, who will step down next week because his age is impairing his ability to govern, is the first pontiff in more than 700 years to formally resign because he can no longer carry on.
There has been only one other pope to resign his duties in similar circumstances in the Church's 2,000-year history, but numerous popes were deposed, exiled or forced to step down.
Four popes died in exile or in prison, six were assassinated, two died of wounds received during riots and one was killed when a roof collapsed.
The only previous pope to step down because he considered himself unable to continue was Celestine V, a simple hermit who was elected against his will in 1294 to end a deadlock among cardinals and resigned after just five months.
Celestine V, or Pietro del Morrone, had been a Benedictine monk, renowned for renouncing worldly goods for a barren life in a mountain cave in the Abruzzo region in central Italy.
When he wrote to the bickering cardinals after the death of Pope Nicholas IV urging them to elect a successor quickly, they chose him.
Shortly after taking up the chair of St. Peter, he issued a decree declaring it possible for a pope to resign, opening up the way for his own departure.
His decision to back out sparked derision and Italian poet Dante Alighieri famously condemned him in The Divine Comedy to spend eternity in hell's antechamber for his "cowardice" in making "the great refusal."
Here are some examples of other popes who have given up the papacy:
-- In 1045, Benedict IX, renowned as one of the most disgraceful popes the Church has known, sold his papacy to his godfather, pious priest John Gratian, so that he could get married. Reportedly unable to persuade the woman in question to have him, he returned to seize the papal seat.
-- In 1046, Gratian, who had reigned shortly as pope Gregory VI and was considered the true pope by many despite Benedict IX's violent return to claim the throne, was forced to resign himself amid accusations he had bought the papacy.
-- In 1415, Gregory XII was forced out as part of a deal to end the "Western Schism", when two rival claimants declared themselves pope in Pisa and Avignon and threatened to tear apart the Church.
-- In 1804, Pius VII signed an abdication of the papal throne before setting out for Paris to crown Napoleon, to be put into effect in case he were imprisoned in France.
-- During the Second World War, Pius XII is reported to have signed a document which said that he was to be considered as having resigned his office should he be kidnapped by the Nazis.
Several popes were booted out or martyred as the Roman Empire collapsed, and the Eternal City -- sacked and depopulated by barbarian hordes -- fell prey to quarrelling factions. But the historical details of how they fell are often lost in the mist of time, and often only the legends remain.