"Strength of mind and body are necessary, strength which in the last few months has deteriorated in me to the extent that I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me," the pope said, according to the Vatican.
Age is also a factor for cardinals. Once a cardinal reaches 80, he is no longer able to participate in the election of the pope or enter the secret conclave where cardinals gather to select the next pope.
Of the 118 cardinals of voting age, 28 are from Italy, 34 are from elsewhere in Europe, 19 are from Latin America, 14 are from the United States and Canada, 11 are from Asia, 11 are from Africa and 1 is from Australia.
Cardinals will meet to choose Benedict's successor sometime after his official resignation on February 28, the Reverend Federico Lombardi, a Vatican spokesman, said at a news conference.
"Before Easter, we will have the new pope," he said.
Benedict won't be involved in the decision, Lombardi said. But his influence will undoubtedly be felt. Benedict appointed 67 the 118 cardinals who will make the decision.
It's a choice that Cardinal Donald Wuerl said he doesn't take lightly.
"When we go into the conclave, what has to be upper in the minds of all of us is what is God asking of us in making a choice. Who will fill the chair of Peter? And I think that's going to be the only consideration," said Wuerl, the archbishop of Washington. "Who among this body has the qualifications, the characteristics, the spiritual gifts to fill that chair?"
Wuerl told reporters that he was in his study at 5 a.m. Monday preparing a homily for Ash Wednesday when he found out about the pope's decision.
"This is very startling," he said. "I was totally unprepared for it."
Wuerl is a top American contender for the papacy, according to Allen. Cardinal Timothy Dolan, the archbishop of New York, could also be considered, but both Americans would be on the "C or D list" of candidates, Allen said.
While only the church's inner circle will know what goes on inside the conclave, bookmakers were quick to set the odds over who will be the top contenders.
Two online betting sites listed Turkson as a favorite Monday. London-based William Hill plc and Dublin-based Paddy Power both gave him 3-1 odds.