Pope Benedict XVI's surprising upcoming resignation is rare, but not unprecedented.
Indeed, Canon 332 of church law envisions the possibility of papal resignation, requiring only that it is "freely made and duly manifested."
The last time a pope resigned was in 1415, when Gregory XII reluctantly gave up the position in order to end the Western Schism between competing papal claimants.
ANALYSIS: Pope Benedict XVI Resigns in Rare Move
The last pope to resign willingly was Celestine V, who accepted the papacy in 1294 at age 85 after having lived as a Benedictine hermit in the Abruzzi mountains.
Celestine stepped down five months later, citing "deficiencies of physical strength" and "longing for the tranquility of the former life" among the reasons for his decision.
However, Celestine V could not enjoy tranquility in his final months. Fearing a schism, his successor Boniface VIII confined the former pope to a fortress. Celestine died there within a year and a half and was probably killed, as a hole found in his skull suggests.
PHOTOS: Pope Benedict XVI's Reign
Although historical evidence is not always clear, historians date the first resignation to as early as 235 A. D.
At that time, following Rome's persecution of Christians, Pope Pontian was arrested and condemned to exile in salt mines in Sardinia. Worried about leaving the church without a head, Pontian resigned on Sept. 28, 235. He did not survive the exile and died of maltreatment soon after.
Benedict XVI stated his declining health is the reason for his resignation - but many popes have been sick during their mandates.
For example, Clement XII was blind for eight years before he died at age 88 in 1740. More recently, John Paul II carried out his mission until the end, despite being afflicted with Parkinson's disease.
Benedict XVI had raised the possibility of resigning already in 2010, when he was interviewed for the book "Light of the World" by German journalist Peter Seewald.
"If a pope clearly realizes that he is no longer physically, psychologically and spiritually capable of handling the duties of his office, then he has a right, and under some circumstances, also an obligation to resign," Benedict said.
Image: A portrait of Celestine V, the last pope to resign willingly in 1294. Credit: Wikimedia Commons.