The pope's spokesman Federico Lombardi is giving a press conference at the Vatican, which you can watch here, or below. It is in Italian.
Lombardi has said the pope's decision "took us by surprise" – indicating even Benedict's closest aides did not know what he was thinking of doing, Reuters reports. His decision left aides "incredulous", the spokesman said.
Benedict will remain "in full charge" of the Catholic church until 7pm GMT on 28 February.
His resignation is fully in compliance with church law, Lombardi has said, and Benedict took the decision freely.
He is not resigning because of any "difficulties in the papacy". Benedict does not fear a schism in the church after he resigns.
Upon resigning, he will go to the papal summer residence near Rome, and then will move to a cloistered residence in the Vatican.
Benedict will not take part in the conclave to elect his successor, Lombardi says.
He expects a new pope to be elected before the end of March.