It is an ancient ritual, steeped in tradition and shrouded in secrecy.
On Wednesday, the conclave to elect a new pope begins.
So what can we expect?
Here's what we know.
When will the conclave start?
The conclave is expected to begin on Wednesday, May 7 at 10am, local time, with a public Holy Mass at St Peter's Basilica.
After the mass, the cardinals will retreat to their rooms next door at Domus Sanctae Marthae, before heading to the Sistine Chapel in the afternoon for the first vote.
This is where every papal election has taken place since 1878.
How long can conclaves take?
The longest conclave lasted 1,006 days — almost three years — back in the 13th century.
Darius Von Guttner, a historian with Australian Catholic University, said this was due to a "political deadlock" between Italian and French cardinals, who wanted the papacy based in their respective countries.
"In the end, the townsfolk got so annoyed they took the roof off the church where the cardinals were meeting, saying, 'Hurry up, otherwise you'll be cold and wet,'" Mr Von Guttner said.
Thankfully, this conclave shouldn't take as long.
The past two papal elections were decided in just two days, after the fourth ballot in 2005, and the fifth ballot in 2013.
Mr Von Guttner said he "wouldn't be surprised" if this year's conclave took slightly longer.
"I suspect that … perhaps on the afternoon of the third day a consensus will be reached," he said.
How does the conclave begin?
Mr Von Guttner said the cardinals will enter the chapel singing an ancient hymn, Veni Creator Spiritus — Come Holy Spirit.
They'll walk in ascending rank — cardinal deacons, cardinal priests, and finally, cardinal bishops.
After this, they will vow to hold the secrets of the conclave process.
The master of papal liturgical celebrations will then announce "extra omnes" or "everyone out".
At that point, all individuals except the cardinal electors and those mandated to be present must exit.
Who will be voting?
About 250 cardinals are alive today, but those aged over 80 are barred from voting.
That leaves 135 eligible cardinal electors this year, but two have already dropped out.
One is too ill to attend and another has excluded himself after being convicted of embezzlement in 2023.
A record 133 cardinals remain — the previous maximum was 115 in 2013.
It is also the most geographically diverse conclave ever, representing 72 countries, many in the Global South.
Mr Von Guttner said it's also the youngest group ever.
"You've got several cardinal electors still in their 50s," he said.
Can people watch the conclave?
Conclaves are highly secretive affairs.
Once they begin, cardinals are "sequestered" — cut off from the outside world.
"It gives them independence to make a choice they want to make," Mr Von Guttner said.
Vatican staff are also sequestered, taking an oath of secrecy ahead of the conclave.
Before the voting starts, Vatican technicians conduct a security sweep of the building.
This includes a final scan for any hidden microphones or cameras in the area.
A false wooden platform is installed to protect the marble floors of the Sistine Chapel, with electronic jammers placed underneath to block any mobile or wi-fi signals.
The high windows in the Sistine Chapel are also boarded up.
What does the voting look like?
The cardinals will likely sit in three "blocks" or ranks, Mr Von Guttner said.
They'll secretly write their choice on a paper bearing the words Eligo in Summum Pontificem — I elect as Supreme Pontiff.
The electors then proceed one at a time, in ascending rank, to a table at the altar where scrutineers preside over three urns.
Before dropping their folded ballot into one of the urns, they'll say:
Testor Christum Dominum, qui me iudicaturus est, me eum eligere, quem secundum Deum iudico eligi debere.
I call as my witness Christ the Lord, who is to judge me, that I choose him whom according to God I judge ought to be elected.
Who counts the votes?
Before each vote, the cardinals draw lots to choose those who will supervise the vote.
This includes three scrutineers, who manage the urns and read each ballot aloud.
Three infirmarii are also selected — they collect ballots from any electors who are sick during the conclave.
And finally, three revisers — they tally the numbers once the "scrutiny" is finished.
How are the votes counted?
Once the voting is complete, the counting begins.
One scrutineer shakes the urn and another removes and counts the ballots.
If the number of papers in the urn doesn't match the number of electors, the whole vote will be void.
But if there are no irregularities, the scrutineers will open each ballot and note down the name, with the last scrutineer reading it aloud.
In the past, the scrutineers would use a needle and thread to form a "chain" of ballots.
"But I'm not quite sure whether this is going to happen this time," Mr Von Guttner said.
The three revisers then double-check the ballots.
There is one vote on the first day, followed by four votes over the next two days — two in the morning and two in the afternoon.
How is a pope chosen?
A two-thirds-plus-one majority vote must be achieved to elect a new pope.
If not, all the papers will be placed in a third urn and burnt in the chapel stove.
At the same time, a chemical is used to send black smoke through the chapel's chimney to indicate to the public that no new pope has been chosen.
If a new pope isn't selected in three days, the cardinals can pause for a day of prayer and informal discussions before continuing.
What happens once a pope is chosen?
Once a two-thirds majority is achieved, the dean of the college, or the most senior cardinal bishop present, will approach the pope-elect and ask:
Acceptasne electionem de te canonice factam in Summum Pontificem?
Do you accept your canonical election as Supreme Pontiff?
If they accept, they will then be asked:
Quo nomine vis vocari?
By what name do you wish to be called?
Mr Von Guttner said it's a deeply personal decision.
Some popes, like Pope Francis, chose the name of a saint.
Others, like Pope Benedict XVI, chose to be named after a past pope they admired.
When does the public find out?
As soon as this happens, a chemical is used to release white smoke from the chapel's chimney to announce to the world that a pope has been elected.
Mr Von Guttner said the cardinals would then traditionally share a short prayer or liturgy and chant Te Deum — a hymn of thanksgiving.
The new pope will then be taken to a chamber named the "Room of Tears".
"The tradition was that … the pope will cry that someone placed such an enormous burden on their shoulders," Mr Von Guttner said.
Here, he changes into papal regalia.
Popes traditionally wear a white cassock, a white zucchetto (skull cap) and a red mozzetta (short, overcoat-like garments).
They used to wear the papal tiara — a large crown encrusted with gems — but that ceased in the mid-19th century.
"Why would you be wearing kilos on your head if you don't have to?" Mr Von Guttner said.
When is the new pope presented to the public?
About an hour after the white smoke is released, the new pope will appear on the balcony of St Peter's Basilica, Mr Von Guttner said.
The proto-deacon of the College of Cardinals will proclaim in Latin:
I announce to you a great joy;
we have a pope:
The most eminent and most reverend lord,
Lord [first name]
Cardinal of the Holy Roman Church [surname]
who has taken the name [papal name].
Traditionally, the new pope will then briefly address the crowd.
"They say something that is very personal. It's not scripted," Mr Von Guttner said.
Pope Francis's first words were simple: "Brothers and sisters, good evening."
He then asked for prayers before blessing the city and the world — a papal address known as "Urbi et Orbi".
"It's usually 10, 15 minutes," Mr Von Guttner said.
Can I watch a live stream of the conclave?
Unless a recording device has illegally made it past the Vatican technicians, no.
But the world's media will have their lenses trained on the Sistine Chapel's chimney and St Peter's Square.
This will be revealed as soon as black smoke (no decision) or white smoke (decision) is released, and global media coverage of the new pope will begin.
