So, we're all shocked at the resignation of the Pope, but what happens next?
We know the procedure for what happens when a Pope dies, but what happens in these circumstances?
We've had 600 years of Popes dying in office.
The last to resign was in 1415 but in fact the last one to resign entirely voluntarily, for personal reasons, was in 1294.
So in one respect, this is new ground.
But actually nothing will change. The Pope will end his duties at eight in the evening of February 28.
And then, it's as if he were dead - the Cardinals will meet in conclave, in secret, as early as the following morning, to begin the procedure of electing the next Pope.
There are 120 of them who have a vote - one of them will be elected.
It could take a day, or days, or a week. They have rounds of voting at the end of which either black smoke appears, meaning "no, we haven't got a winner yet" or the famous white smoke rises and shortly afterwards a Cardinal emerges to announce "habemus Papam" - we have a Pope. As for who that might be; there's been talk of the first black Pope since the fifth century.
It's not impossible.
There's a Cardinal from Ghana - Peter Appiah Turkson, who is seen as a great communicator, a superstar among cardinals. He is very well regarded and is one of the favourites.
The electoral process is about two things.
The first is the candidate's doctrinal soundness, intelligence, leadership and communication skills.
But it's also about power centres within the Vatican.
Which candidate has their ear? Which Cardinal has the fewest enemies?
This Pope has elevated 67 of the 120 cardinals.
This Curia is in his image.
His successor will almost certainly carry on his values.
More than half are European - in fact this month he elevated 22 Archbishops, most of them European.
28 are Italian. The whole of Latin America has 21, Africa just 11.
So Italians are among the favourites - especially the Archbishiops of Milan and Genoa.
There's also a very popular and able Canadian.
The present Pope was old even when he was elected, a stop gap, a conservative leader of a church that has grown increasingly defensive and wary.
The next one may be younger, more of a communicator and, whoever he is, he'll be in place by Easter.