Cardinals are cocooned in the Vatican
during the secret voting process
Papal conclave: Runners and riders By Mike Wooldridge & Michael Hirst BBC News, London
Pope Benedict XVI's successor will be chosen by 115 cardinal-electors during a secret election - known as a Conclave - in the Vatican's Sistine Chapel.
Canon Law states that any male who has been baptised is eligible to be elected, but since the late 14th Century the Pope has come from this body of Princes of the Church.
A post once almost exclusively held by Italians has most recently been filled by a Pole and a German, so the race is open, although the composition of the electors offers clues to who might be a frontrunner for the papacy - or papabile.
A two-thirds-plus-one vote majority is required, meaning the man elected is likely to be a compromise candidate. Sixty-seven of the electors were appointed by Benedict XVI, and 50 by his predecessor John Paul II.
About half the cardinal-electors (60) are European - 21 of those being Italian - and many have worked for the administrative body of the Church, the Curia, in Rome.
Thus, a candidate's credentials will be bolstered if he has Curial experience and affinity with Europe - a working knowledge of Italian is seen as a prerequisite.
But there is also speculation the new pontiff may come from one of the Church's growth areas - 42% of the world's 1.2bn Catholics come from Latin America, as do a sixth of the electors.
Here is a selection of the leading papabili.
Angelo Scola, Italy
Cardinal Angelo Scola, 71, is the most prominent Italian candidate and has been referred to by one Catholic newspaper as the "crown prince of Catholicism".
A cardinal since 2003, he was appointed Archbishop of Milan in 2011. Cardinal Scola is a conservative, who has been close to both John Paul II and Pope Benedict, both personally and theologically.
In 2010, at the height of sex abuse allegations against the church, he called the media's attacks on the Pope "an iniquitous humiliation".
Given Pope Benedict's reasons for resigning, however, it is possible his relatively advanced age may stand against him.
Marc Ouellet, Canada
Cardinal Ouellet, 68, from Canada, has headed the Congregation for Bishops since 2010 and has strong Curial connections.
A native French speaker who also speaks fluent Spanish, he has spent much of his life since ordination as a seminary professor and rector, spending 10 years in Colombia and nine in Canada before being appointed to teach at the John Paul II Institute for Marriage and Family in 1997.
A former editor of Communio, an international journal co-founded by Joseph Ratzinger, his thinking is closely linked with that of the resigning Pope. He also has close connections with the Latin American Church.
After a brief stint as vice-president of the Pontifical Council for Promoting Christian Unity, he was named Archbishop of Quebec in 2002 and appointed a cardinal in 2005. Since then, he has stoked controversy by speaking out on moral issues in Canada's largely secular society.
Christoph Schoenborn, Austria
Cardinal Schoenborn, Archbishop of Vienna, is probably the strongest non-Italian candidate from within Europe.
The son of a Bohemian count, he was born in 1945 to a family with a long history of high office in the Catholic church and the Holy Roman Empire.
He was made a cardinal in 1998 and, although seen as intellectually conservative, in 2010 he caused controversy by suggesting it was time to re-examine the issue of priestly celibacy.
Cardinal Schoenborn later issued a clarification, saying he was not "seeking to question the Catholic Church's celibacy rule".
Odilo Scherer, Brazil
The archbishop of Sao Paulo, Cardinal Odilo Scherer, 63, is the most prominent Latin American candidate.
While head of the largest diocese in the world's largest Catholic country, Brazil, Cardinal Scherer has also gained considerable Vatican credentials.
He obtained his doctorate in theology from the Pontifical Gregorian University in Rome, and worked at the Congregation for Bishops there.
He has been seen as a compromise candidate who could satisfy both European and Latin American congregations. On the other hand, the 63-year-old German-Brazilian has not been able to reverse a marked downward trend in the number of Catholics in Latin America.
Leonardo Sandri, Argentina
Cardinal Sandri, 63, was born in Buenos Aires in Argentina to Italian parents.
He became a papal diplomat after ordination and served as apostolic nuncio to Venezuela and Mexico.
Between 2000 and 2007 he was third-in-command at the Vatican, serving as its chief of staff.
Towards the end of John Paul II's papacy, he became the ailing pope's spokesman, and it was Cardinal Sandri who delivered the announcement of the Pope's death in St Peter's Square 2005.
He now heads the Vatican department for Eastern Churches.
Peter Turkson, Ghana
Born in western Ghana in October 1948 to a Methodist mother and a Catholic father, Peter Turkson became the first-ever Ghanaian cardinal in 2003 when he was appointed by Pope John Paul II.
The 64-year-old is the relator, or general secretary, of the Synod for Africa, making him a strong candidate to become the first African pope of the modern age, taking on a mantle that was held during the 2005 Conclave by Nigerian Cardinal Francis Arinze.
The last two Popes both served as relators for a synod of bishops.
Cardinal Turkson is also the head of Vatican's Council for Justice and Peace, which released a document in 2011 calling for radical economic reforms to deal with the global recession.
The document condemned the "idolatry of the market", and Cardinal Turkson expressed support for the Occupy Wall Street protest movement.
Theologically, he is seen as a moderate, signalling openness, for example, to the argument that condoms might be appropriate for couples where one partner is HIV-positive and the other is not.
In a BBC interview on Monday, Cardinal Turkson side-stepped a question about whether he could be the next pontiff.
Luis Tagle, Philippines
At 55, Luis Tagle is one of the youngest papabili or potential candidates.
He is archbishop of the Philippines' capital city, Manila - a 2.8 million-strong archdiocese, and he was made a cardinal only a few months ago, in November 2012.
Cardinal Tagle has gained a reputation as a man of the people - as bishop, he is once said to have ridden a cheap bicycle to a run-down neighborhood in Manila, to deputise for a sick colleague.
He's also known for inviting beggars outside his cathedral to share a meal with him. Tagle is one of the more media-savvy cardinals. He is a frequent broadcaster in the Philippines and has a presence on Facebook.
Joao Braz de Aviz, Brazil
The 65-year-old from Brazil has had his reputation bolstered since taking over as prefect of the Congregation for Institutes of Consecrated Life and Societies of Apostolic Life in 2011.
One of eight children, he was born in Mafra, Santa Catarina, and completed his theological studies at Rome's Pontifical Gregorian and Pontifical Lateran Universities.
As a young parish priest in Brazil he was caught in the cross-fire of an armed robbery, with bullets perforating his lungs, intestines and an eye: some bullet fragments remain lodged in his body.
Having been made a bishop in 1994 he was appointed archbishop of Brasilia in 2004 and in May 2010 he organised the XVI National Eucharistic Congress to coincide with the 50th anniversary of the city.
He has focused on the welfare of the poor as espoused by the Liberation Theology popular in Latin America. But he distances itself from its ideological "excesses", saying it almost caused him to abandon his vocation.
Timothy Dolan, United States
Cardinal Dolan, 62, from the United States, is the archbishop of the influential New York archdiocese.
He has extensive pastoral experience, having headed the Milwaukee diocese before that.
An affable character who has also ably led the US conference of Catholic bishops, he also has strong theological credentials with a PhD in Church History and spent in Rome both as a student and as rector of the North American College.
However, the very fact that he is American may stand in his way - cardinals are generally seen as reluctant to promote figures from a perceived super power state.
Gianfranco Ravasi, Italy
Cardinal Ravasi, 70, has been the president of the Pontifical Council for Culture - or the Vatican's culture minister - for the past five years and so has strong Curial and academic credentials.
His biblical scholarship has helped him popularise Scripture studies through Italian television, radio and popular magazines.
Some might see this as a disadvantage if electors seek to promote a pastor rather than a professor
Before he moved to Rome he was a professor and director of the Ambrosian Library in his native Milan - a highly-regarded hub of theological scholarship.
A European intellectual seen as a "moderate" ecclesiologically, he is perhaps seen as lacking global experience.
