There are several papal contenders in the wings, but no obvious front-runner — the same situation when Benedict was elected pontiff in 2005 after the death of Pope John Paul II.
When Benedict was elected pope at age 78 — already the oldest pope elected in nearly 300 years — he had been already planning to retire as the Vatican's chief orthodoxy watchdog to spend his final years writing in the "peace and quiet" of his native Bavaria.
Contenders to be his successor include Cardinal Angelo Scola, archbishop of Milan, Cardinal Christoph Schoenborn, the archbishop of Vienna, and Cardinal Marc Ouellet, the Canadian head of the Vatican's office for bishops.
Longshots include Cardinal Timothy Dolan of New York. Although Dolan is popular and backs the pope's conservative line, the general thinking is that the Catholic Church doesn't need a pope from a "superpower."
All cardinals under age 80 are allowed to vote in the conclave, the secret meeting held in the Sistine Chapel where cardinals cast ballots to elect a new pope. As per tradition, the ballots are burned after each voting round; black smoke that snakes out of the chimney means no pope has been chosen, while white smoke means a pope has been elected.
Popes are allowed to resign; church law specifies only that the resignation be "freely made and properly manifested."
Only a handful have done so, however and there's good reason why it hasn't become commonplace: Might the existence of two popes — even when one has stepped down — lead to divisions and instability in the church? Might a new resignation precedent lead to pressures on future popes to quit at the slightest hint of infirmity?
Benedict himself raised the possibility of resigning if he were simply too old or sick to continue on in 2010, when he was interviewed for the book "Light of the World."
"If a pope clearly realizes that he is no longer physically, psychologically and spiritually capable of handling the duties of his office, then he has a right, and under some circumstances, also an obligation to resign," Benedict said.
The former Cardinal Joseph Ratzinger had an intimate view as Pope John Paul II, with whom he had worked closely for nearly a quarter-century, suffered through the debilitating end of his papacy.