Ever since Benedict XVI's shock resignation, Cathoilic cardinals have been debating the challenges that the next pope will face and vetting possible candidates for the post, as well as airing rare criticism of the Vatican's opaque bureaucracy.
Vatican insiders put Milan Archbishop Angelo Scola in the lead, but short of the two-thirds of the 115 "cardinal electors" needed to become the new leader of the world's 1.2 billion Catholics.
Brazilian Odilo Scherer, the charismatic archbishop of Sao Paolo and Latin America's best hope, is also seen as in with a chance after the red-frocked cardinals begin the storied process, cloaked in secrecy, of choosing one of their peers to lead the Church.
The electors must take a solemn oath of secrecy or face excommunication -- though no examples of such a fate appear in the record.
"In order of protocol, they will approach the lectern and pronounce the formula of the oath one by one, placing a hand on the Bible," Vatican spokesman Federico Lombardi said.
Some 90 staffers including cooks, drivers and security guards who will work around the conclave from Tuesday took the same oath already on Monday.
Cardinal Tarcisio Bertone, in his capacity as the Vatican camerlengo, or chamberlain, oversaw that ceremony in the Vatican's Paolina Chapel.
Lombardi said some 5,600 media professionals have been accredited to cover the event, which kicks off with a grand mass co-celebrated by the cardinals in St Peter's Basilica on Tuesday at 0900 GMT to invoke divine assistance in their deliberations.
Lombardi described the complex protocol by which the cardinals -- all 115 of whom were appointed by Benedict or his predecessor John Paul II -- will later file into the Sistine Chapel.
Then with the words "extra omnes" (everyone out), the chapel's great wooden doors will swing shut and the cardinal electors will be left to their task.
The last non-cardinal to leave the chapel will be the prefect of the papal household, Georg Gaenswein -- who is also Benedict XVI's personal secretary.
Lombardi said the first round of voting Tuesday was unlikely to produce a new pope, and told journalists to expect black smoke to emerge from the chimney near the Sistine Chapel at around 8:00 pm (1900 GMT).
In a tradition going back centuries, the cardinals inform the world of a successful election by sending up white smoke, followed soon after by the famous phrase, "Habemus Papam" (we have a pope).
Meanwhile all is ready in the Sistine Chapel, which will be swept daily for listening devices to keep would-be spies at bay. Communications around the chapel and the cardinals' nearby lodging will be jammed.
The cardinals are looking for a pope strong enough to grapple with the challenges assailing the Catholic Church that proved too much for 85-year-old Benedict.
His resignation -- the first for 700 years -- has focused attention on the need to find a leader with the energy to shape the Church's approach to growing secularism in the West and the Islamic radicalism spreading to many parts of the globe.
Cardinals have expressed a desire for a more vigorous, pastoral figure to deal with the relentless scandals over sexual abuse by paedophile priests and cover-ups by superiors that have rocked the Catholic Church.
"Critics would say the most important piece of unfinished business" is cracking down on bishops who protect paedophile priests, said Vatican expert John Allen of the National Catholic Reporter.
"The Vatican now has tough policies against priests who abuse, but it does not have equally tough policies for bishops," Allen told AFP.
The cardinals also want a man who can reform the Roman Curia, the central government of the Catholic Church, which has been beset by the intrigue laid bare in documents leaked by Benedict's butler last year.
While Canadian Marc Ouellet has attracted attention as the powerful prefect of the Congregation of Bishops, Scola was thronged by photographers and cameramen when he celebrated mass in the Church of the Twelve Holy Apostles on Sunday.
Scola, 71, a hardliner cut from the same cloth as Benedict, the German-born arch-conservative Joseph Ratzinger, has the advantage of not being associated with the tarnished Vatican bureaucracy.
The conclave, named after the Latin phrase meaning "with key", comes eight centuries after the first lock-in, when cardinals who had dithered over their choice for nearly three years were given just bread and water until they finally made up their minds.
Another leading contender, according to Italian media, is American prelate Timothy Dolan -- even though the cardinal told one interviewer that anyone mentioning him as a candidate must be "smoking marijuana".