Pope Benedict XVI is to resign at the end of the month, at the age of 85.
He is the first pontiff to have stepped down since Gregory XII in 1415.
Canon Law states: "If it happens that the Roman Pontiff resigns his office, it is required for validity that the resignation is made freely and properly manifested but not that it is accepted by anyone."
Pope Benedict's resignation has set in motion the centuries-old process of electing a new pope.
Cardinals summoned to Rome
Popes are chosen by the College of Cardinals, the Church's most senior officials, who are appointed by the Pope and usually ordained bishops. They are summoned to a meeting at the Vatican which is followed by the Papal election - or Conclave.
There are currently 203 cardinals from 69 countries. The rules of the Conclave were changed in 1975 to exclude all cardinals over the age of 80 from voting. The maximum number of cardinal electors is 120.
During the forthcoming Conclave, there will be 115 cardinal-electors: they have to be younger than 80 to be eligible to vote, but Cardinal Julius Riyadi Darmaatmadja, the 78 year-old Archbishop Emeritus of Jakarta, has ruled himself out of travelling to Rome due to the "progressive deterioration" of his vision.
Cardinal Keith O'Brien - Britain's most senior Catholic cleric - has also been ruled out of the voting after his resignation over allegations of inappropriate conduct.
Normally the Dean of the College of Cardinals would be responsible for the convoking the Conclave. However, as the Dean, Italian Cardinal Angelo Sodano, is 85 and too old to vote, the senior cardinal-elector, Giovanni Battista Re, takes on the responsibility.
Sixty-seven of the men who will vote for the new pope were appointed by Benedict XVI, and 49 by his predecessor John Paul II. About half (60) are European, and 21 are Italian. There will also be 19 Latin Americans, 14 North Americans, 11 Africans, 10 Asians and one cardinal from Oceania among the voters.
During the time between the Pope's resignation and the election of his successor, the college of cardinals will govern the Church, headed by Cardinal Tarcisio Bertone, as the cardinal camerlengo - or chamberlain.
It is his job to supervise the whole election process, with secret votes being held four times daily inside the Vatican's Sistine Chapel. During the Conclave, cardinals reside within the Vatican and are not permitted any contact with the outside world.
During this period all the cardinals - retirees included - will begin to discuss in strict secrecy the merits of likely candidates.
The cardinals do not have to choose one of their own number - theoretically any baptised male Catholic can be elected pope - but tradition says that they will almost certainly give the job to a cardinal.
The Vatican talks about the cardinals being guided by the Holy Spirit. But although open campaigning is forbidden, a papal election is still a highly political process.
The coalition-builders have about two weeks to forge alliances and senior cardinals who may themselves have little chance of becoming pope can still exert a considerable influence over the others.
The election of a pope is conducted in conditions of secrecy unique in the modern world.
The cardinals are shut away in the Vatican until they reach agreement - the meaning of the word conclave indicating that they are literally locked up "with a key".
The election process can take days. In previous centuries it has gone on for weeks or months and some cardinals have even died during conclaves.
The process is designed to prevent any of the details of the voting emerging, either during or after the conclave. The threat of excommunication hangs over anyone tempted to break this silence.
John Paul II changed the rules of the Conclave so a Pope could be elected by simple majority.
But Benedict XVI changed the requirements back so that a two-thirds majority is required, meaning the man elected is likely to be a compromise candidate.
Before the voting begins in the Sistine Chapel, the entire area is checked by security experts to ensure there are no hidden microphones or cameras.
Once the conclave has begun, the cardinals eat, vote and sleep within closed-off areas until a new pope has been chosen.
They are allowed no contact with the outside world - barring a medical emergency. All radios and television sets are removed, no newspapers or magazines are allowed in, and mobile phones are banned.
Two doctors are allowed into the conclave, as well as priests who are able to hear confessions in various languages and housekeeping staff.
All these staff have to swear an oath promising to observe perpetual secrecy, and undertake not to use sound or video recording equipment.
Voting is held in the Sistine Chapel, "where everything is conducive to an awareness of the presence of God, in whose sight each person will one day be judged".
On the day the conclave begins, the cardinals celebrate Mass in the morning before walking in procession to the chapel.
Once the cardinals are inside the conclave area, they have to swear an oath of secrecy. Then, the Latin command "extra omnes" ("everyone out") instructs all those not involved in the election to leave before the doors are closed.
The cardinals have the option of holding a single ballot on the afternoon of the first day. From the second day, two ballots are held in the morning and two in the afternoon.
The ballot paper is rectangular. Printed on the upper half are the words "Eligio in Summum Pontificem" ("I elect as Supreme Pontiff"). Below is a space for the name of the person chosen. The cardinals are instructed to write the name in a way that does not identify them, and to fold the paper twice.
After all the votes have been cast, the papers are mixed, counted and opened.
As the papers are counted, one of the scrutineers calls out the names of those cardinals who have received votes. He pierces each paper with a needle - through the word "Eligio" - placing all the ballots on a single thread.
The ballot papers are then burned - giving off the smoke visible to onlookers outside which traditionally turns from black to white once a new pope has been chosen.
Damp straw was once added to the stove to turn the smoke black, but over the years there has often been confusion over the colour of the smoke. More recently a dye has been used.
If a second vote is to take place immediately, the ballots from the first vote are put on one side and then burned together with those from the second vote. The process continues until one candidate has achieved the required majority.
Reaching a decision
Pope John Paul II changed the rules of election in 1996. Previously, a candidate had to secure a majority of two-thirds to be elected pope (two-thirds plus one vote if the number of cardinals does not divide by three).
John Paul II ruled that the voting could shift to a simple majority (50% plus one vote) after about 12 days of inconclusive voting.
In 2007, Pope Benedict passed a decree reverting back to the two-thirds majority, thus encouraging cardinals to reach consensus, rather than one bloc backing a candidate with more than half the votes and then holding out for 12 days to ensure his election.
If after three days of balloting nobody has gained the two-thirds majority, voting is suspended for a maximum of one day to allow a pause for prayer, informal discussion and what is described as "a brief spiritual exhortation" by the senior cardinal in the Order of Deacons.
At the end of the election, a document is drawn up giving the results of the voting at each session, and handed over to the new pope. It is kept in an archive in a sealed envelope, which can be opened only on the orders of the pope.
The only clue about what is going on inside the Sistine Chapel is the smoke that emerges twice a day from burning the ballot papers. Black signals failure. The traditional white smoke means a new pope has been chosen.
New pope announced
After the election of the new pope has been signalled by white smoke rising from the Sistine Chapel chimney, there will be a short delay before his identity is finally revealed to the world.
Once one candidate has attained the required majority, he is then asked: "Do you accept your canonical election as Supreme Pontiff?"
Having given his consent, the new pope is asked: "By what name do you wish to be called?"
After he has chosen a name, the other cardinals then approach the new pope to make an act of homage and obedience.
The new pope also has to be fitted into his new robes. The papal tailor will have prepared garments to dress a pope of any size - small, medium or large - but some last-minute adjustments may be required.
Then, from the balcony of St Peter's Basilica, the traditional announcement will echo around the square: "Annuntio vobis gaudium magnum... habemus papam!" - "I announce to you a great joy... we have a pope!"
His name is then revealed, and the newly-elected pontiff will make his first public appearance.
After saying a few words, the pope will give the traditional blessing of Urbi et Orbi - "to the city and the world" - and a new pontificate will have begun.