The 85-year-old pontiff made the announcement Monday, saying he no longer had the strength to carry out his papal duties.
"After having repeatedly examined my conscience before God, I have come to the certainty that my strengths, due to an advanced age, are no longer suited to an adequate exercise of the Petrine ministry," the pope said according to a statement released by the Vatican.
He said he is aware of the "seriousness" of his resignation, but that he did so in "full freedom."
The last pope to resign was Pope Gregory XII, who stepped down in 1415 in a deal to end the Great Western Schism among competing papal claimants.
Born Joseph Aloisius Ratzinger, Pope Benedict became the 265th pope after being elected by his fellow Cardinals elected on April 19, 2005 at the age of 78 following the death of the popular John Paul II. The ninth German pope and first in nearly 500 years, Benedict was ordained in 1951 and was a major figure at the Vatican stage for decades before his ascension.
Ratzinger chose the name Benedict, which comes from the Latin word meaning "the blessed," in homage to Pope Benedict XV, who was pope during World War I, and Saint Benedict of Nursia, who established the Benedictine monasteries.
The sudden announcement sets the stage for the Vatican to hold a conclave to elect a new pope by mid-March, since the traditional mourning time that would follow the death of a pope doesn't have to be observed. There are several papal contenders in the wings, but no obvious front-runner, according to Vatican watchers.
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POPE BENEDICT'S RESIGNATION STATEMENT
HOW A POPE IS ELECTEDDear Brothers,
I have convoked you to this Consistory, not only for the three canonizations, but also to communicate to you a decision of great importance for the life of the Church.
After having repeatedly examined my conscience before God, I have come to the certainty that my strengths, due to an advanced age, are no longer suited to an adequate exercise of the Petrine ministry.
I am well aware that this ministry, due to its essential spiritual nature, must be carried out not only with words and deeds, but no less with prayer and suffering.
However, in today’s world, subject to so many rapid changes and shaken by questions of deep relevance for the life of faith, in order to govern the bark of Saint Peter and proclaim the Gospel, both strength of mind and body are necessary, strength which in the last few months, has deteriorated in me to the extent that I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me.
For this reason, and well aware of the seriousness of this act, with full freedom I declare that I renounce the ministry of Bishop of Rome, Successor of Saint Peter, entrusted to me by the Cardinals on 19 April 2005, in such a way, that as from 28 February 2013, at 20:00 hours, the See of Rome, the See of Saint Peter, will be vacant and a Conclave to elect the new Supreme Pontiff will have to be convoked by those whose competence it is.
Dear Brothers, I thank you most sincerely for all the love and work with which you have supported me in my ministry and I ask pardon for all my defects.
And now, let us entrust the Holy Church to the care of Our Supreme Pastor, Our Lord Jesus Christ, and implore his holy Mother Mary, so that she may assist the Cardinal Fathers with her maternal solicitude, in electing a new Supreme Pontiff.
With regard to myself, I wish to also devotedly serve the Holy Church of God in the future through a life dedicated to prayer.
Roman Catholic cardinals seeking a successor to Pope Benedict XVI will hold a conclave to elect a new pontiff.
Only cardinals are eligible to take part in the conclave, which will continue until a successor is chosen.
The cardinals will meet in the Vatican's ornate Sistine Chapel and hold two voting rounds a day until they choose a new pope with a two-thirds majority.
They were traditionally locked into the Chapel, best known for the frescoed ceiling and altar wall painted by Michelangelo, and not allowed out until they chose a new pontiff.
They had to sleep in makeshift cells and share minimal sanitary facilities.
But new regulations issued by Pope John Paul II in 1996 allow them to live in a new hotel built on Vatican grounds behind St. Peter's Basilica and even take walks in the tiny state's peaceful gardens between their voting rounds.
Another reform lets the cardinals opt for a simple majority vote if they have not succeeded in electing a pope after about two weeks of balloting.
Most modern conclaves have lasted only a few days.
When the cardinals are in agreement, the chosen one will say 'Accepto,' a puff of white smoke will emerge from the chimney, bells will toll and a cardinal will appear at the central window of St Peter's Basilica to declare 'Habemus papam' - 'We have a pope'.