MR. ALLEN: The Cardinals are not supposed to be casting votes based on their image or based on political considerations, but based on who they really think is best for the Church. And the notion is that doing that behind closed doors makes that somehow easier, makes that more possible.

LAWTON: Sequestered inside the Sistine Chapel, the Cardinals vote by paper ballot, guided, the church says, by the Holy Spirit.

FATHER REESE: They have a small piece of paper and on it they write the name of the person they're voting for. Then they fold that piece of paper in two and hold it in their hand and march up one by one, holding it in the air so that everyone can see there is only one ballot here.

LAWTON: Selected "Cardinal-scrutineers" count the number of ballots, making sure they correspond with the number of cardinals in the room. They then tally the ballots aloud. Under previous rules, the Pope was always chosen by a two-thirds vote. There can be four votes a day. After three days, the voting can be suspended for a day of further prayer and discussion. Technically, any baptized male can be elected pope, although since the 14th century, he has come from the College of Cardinals.

In 1996, Pope John Paul II added a new twist to the balloting. It's complicated but if a new pope hasn't been elected after up to 34 votes over ten or 12 days, the Cardinals have the option of moving to a simple majority vote.

After each round of voting, the ballots are burned in a special stove that has been used since the beginning of the 20th century.

FATHER REESE: If the ballot had not elected a pope, they would put chemicals in to make the smoke black. If a pope is elected, they put certain chemicals into the stove with the ballots, so that the smoke comes out white. During the last conclave, they had some problems because the man in charge didn't quite know how to put the chemicals in, and so the smoke was coming out gray, and everybody was scratching their heads saying, "Do we have a pope or don't we?"

LAWTON: Outside, people gather in St. Peter's Square to pray and to await the word from the conclave.

Father Gerald O'Collins was in Rome in 1978 when Pope John Paul II was elected. The rector of Rome's Gregorian University announced the news after Mass.

FATHER O'COLLINS: At the end of the mass he said, "Boys ... you know there's white smoke." So everybody rushes up to St. Peter's. A huge crowd there. The people in Rome get into the spirit of that kind of thing. They just take to the streets and the cars have to stop. I wouldn't have missed it for the world.

LAWTON: In the past, conclaves were sometimes held outside Rome. The process itself has evolved significantly over the centuries.

The conclave system was first institutionalized in the late 13th century here in Viterbo, north of Rome. Pope Gregory IX set down a set of rules for the future after the lengthy -- and chaotic -- process of his own election. This palace was the first place the cardinals were literally locked into a conclave.

In the year 1268, 18 Cardinals gathered here to elect a new pope. But they were deeply divided and distracted by political tensions. After a year and a half of unsuccessful negotiations, local authorities decided to help the Cardinals focus by locking them into the palace. Soon, even more drastic measures were needed.

MONSIGNOR SALVATORE DEL CIUCO (San Lorenzo Cathedral, Viterbo): It was still taking months to choose a pope, so the people of Viterbo asked the Cardinals, "Why don't you hurry up and choose a pope?" The Cardinals said, because the descent of the Holy Spirit on our heads is not easy. So the people said, "Let's facilitate the descent of the Holy Spirit. If we remove the roof of the palace he will come down easier." And in fact, in a few days, the new pope Gregory IX was elected.

LAWTON: Modern popes have made their own changes that could have a huge impact on the future. In 1975, Paul VI instituted the age limit of 80 and expanded the number of voting cardinals.

John Paul II has further expanded and internationalized the College of Cardinals.

ALLEN: The odds of a pope who is not European and not Italian are much more than they ever have been, simply because numerically, the blocs from those non-European places are much larger and therefore have the political capacity to put forward their candidates.

LAWTON: John Paul II's option of moving to a majority vote may also have significant implications. Some say this will streamline the process. Others believe it may have the opposite effect, encouraging small factions of cardinals to hold out for a simple majority, rather than seeking compromise.

FATHER REESE: The larger a group, the more difficult it is for the group to come to consensus.

LAWTON: Another new factor for the next conclave is the proliferation of the media, which will challenge the vow of secrecy, and perhaps also shape the choices.

REESE: It's going to have to be someone who is at ease being the center of attention with the media. That's just part of the reality of being pope today, whether the Church likes it or not.

LAWTON: Indeed, many pundits have drawn up lengthy lists of the qualifications for the next pope, as they have in years past. Not everyone thinks that is a helpful exercise.

O'COLLINS: He had to speak English. He had to speak Spanish. He had to speak other languages. He had to be holy. He had to be learned. You had to have all these experiences. They had this enormous list of the characteristics of the next pope and I thought, "Jesus Christ himself couldn't satisfy that." I mean, let the Pope be the pope he's going to be.

LAWTON: Inevitably, observers are making up their short lists of candidates -- lists that have already been revised many times. But, the election of both John Paul I and John Paul II came as surprises.

FATHER O'COLLINS: People do talk about the Holy Spirit being at the conclave, and I thought, that's interesting. It's a kind of almost hint of that. The obvious candidates that journalists and people like myself, the self-appointed pundits think about, they're not the ones who are elected.

LAWTON: And beyond the politics and the process and the pageantry, for the world's Catholics, the next conclave will ultimately be a holy endeavor.

I'm Kim Lawton in Rome.