"It was a bolt out of the blue," Italian cardinal Angelo Sodano, the dean of the College of Cardinals, said.
NEWS: Pope Brings Twitter Closer to God
The number popes who have resigned has been estimated as high as 10, but the evidence is limited. Most recently (although it was hardly recent at all) Gregory XII resigned at the end of the Western Schism and a new pope was elected in 1417.
Born on April 16, 1927, in Marktl am Inn, Germany, as Joseph Ratzinger, Benedict XVI is said to suffer from joint pain and rheumatism.
Last year he started using a cane and recently appeared to have trouble reading the text of an address he delivered in Rome.
The Pope is also suffering from chronic atrial fibrillation, but refuses to take his prescribed anticoagulant medication, his doctors said.
A top-secret, leaked Vatican document, published last year by the Italian daily Il Fatto Quotidiano, predicted the Pope's death by November 2012.
The anonymous report revealed that Benedict XVI was secretly working on his succession, having chosen Milan cardinal Angelo Scola as the next Pope.