It doesn't happen very often, a Pope resigning. Only once before in fact, voluntarily and for personal reasons, in 1294, when Pope Celestine 5th left after just five months.
Although Gregory 12th resigned in 1215 it was only to end a Schism in the Church. Other Popes were eased out, unwillingly.
This one came out of a clear blue sky. Pope Benedict the Sixteenth had said just over a year ago he wouldn't leave the Papacy soon because of serious issues, like the abuse of young people by priests around the world.
But he was not in good health and he is not a young man - he'll be 86 in April.
The Vatican says he will resign on February the 28th. After that the Cardinals will meet in conclave and next month, the world's Catholics will have a new leader.
The choice of the then Cardinal Ratzinger, a German, was no surprise. He was the much loved John Paul 2nd's right hand man.
He was known as "God's Rottweiler"; a tough disciplinarian who would continue the Polish Pope's doctrinal hard line.
Ratzinger became Pope at 78, having overcome controversy about his membership of the Hitler youth organisation at 14, a requirement of young Germans at the time. Now the Cardinals will elect a new man; most likely a younger bishop from their ranks.
Pope Benedict said his strength was "no longer adequate to continue".
Within a month, the world will have a new Pope.