The centuries-old ritual begins when all the cardinals swear a solemn oath of secrecy at 1545 GMT and hold a first vote to find a new leader for the 1.2 billion Catholics around the globe.
Beyond the Renaissance symbolism and rules that cut off the cardinals from any contact with the outside world during the conclave, the challenge will be to find a 266th pope able to deal with the many challenges assailing the Church.
The abrupt end to Benedict's often troubled pontificate has brought into sharp relief the need for a new kind of leader able to face growing secularism in the West and rising Islamic radicalism in many parts of the world.
Cardinals have called for a more vigorous, pastoral figure who can also overhaul the intrigue-filled Roman Curia, the central government of the Church, and deal with the ongoing scandal over sexual abuse by paedophile priests and cover-ups.
The calls for transparency have intensified since last year when hundreds of confidential papal documents were leaked by Benedict's butler Paolo Gabriele in a scandal known as "Vatileaks" that revealed infighting inside the Curia.
The tradition of priestly celibacy and the calls for an easing of the Catholic Church's hardline stance against artificial contraception and same-sex unions will also be high on the next pope's agenda.
Even seasoned Vatican observers admit that the field is still wide open. "There is no strong candidate who has yet emerged," said Marco Politi, an Italian expert on the Vatican and author of a biography of Benedict.