Cardinals from 48 nations are gathering in the Sistine Chapel at the Vatican, where they will deliberate and vote for a new Pope.
The secret conclave started at 3.30pm UK time and could last for several days, until two thirds can agree on a candidate.
They will be locked in the chapel during the process and are sworn to secrecy. There is no clear front-runner to lead 1.2 billion Catholics worldwide. Speaking from Rome, Austen Ivereigh from the group Catholic Voices told Premier's Marcus Jones during the News Hour the race is wide open:
This morning in preparation for the conclave the 115 cardinal-electors attended a special mass in St Peter's Basilica.
In his homily, the Dean of the College of Cardinals, Cardinal Angelo Sodano, called for unity and said: "We implore the Lord, that through the pastoral sollicitude of the Cardinal Fathers, He may soon grant another Good Shepherd to his Holy Church."
Catholic Commentator Father James Cassidy tells Premier more about what the cardinals will be doing:
During the conclave the cardinals live together and share rooms and are forbidden to watch television, listen to the radio or have any access to the internet. US cardinal Timothy Dolan sent a tweet just a few hours before entering the conclave, saying: "Greetings again from Rome, the Eternal City, the city of Saints Peter and Paul!" Mary Hollingsworth is author of The Papal Conclave and tells Premier why it's such a secretive process:
The first smoke signal from the locked-in cardinals is due at about 6pm UK time today. If it is black this means no successor has been chosen.
If it is white it means the 266th Bishop of Rome has been selected. Commentators predict today's smoke will be black.
From Wednesday, two votes will be held each morning and afternoon and the ballots will be burned after each session. Premier's Maria Rodrigues-Toth is in Rome and says hard to predict who'll be chosen as the next pontiff:
Benedict XVI stepped down last month saying he didn't feel he was mentally or physically capable of leading the Church. He was the first Pope to resign in 600 years and it has thrown the Catholic church into turmoil and exposed deep divisions. A new survey of 2,000 adults showed child abuse within the Catholic Church should be the first issue the new Pope tackles The ComRes poll for Premier Christian Radio also found that improving the Church's reputation should be high on the pontiff's agenda.
Gay marriage, contraception, abortion and assisted dying emerged as less important issues.
Chief Executive Peter Kerridge said:
"There is strong agreement among the British public that improving the image of the Catholic Church should be high on the list of priorities for the new Pope.
"And, no doubt reflecting media headlines, child abuse within the Church is most likely to be seen as the single most important issue confronting him.
"Perhaps surprisingly, issues such as gay marriage, contraception, celibacy of priests, abortion and women priests are not seen by a large proportion of the public to be the most important issues facing the Catholic Church, suggesting that, from the public's viewpoint, the focus of the Church has maybe been directed towards the wrong issues."