Here's from the Guardian:

These are the main reasons the pope gives for his resignation in his statement this morning:

• His "advanced age" (he is 85) means he is no longer able to carry out all his tasks adequately.

• Both strength in mind and body are necessary for the job of pope, he says, and in the last few months such strength has deteriorated in him.

He also announced in the statement that he would renounce the office of Bishop of Rome at 8pm on 28 February.

He says a conclave will have to be convened to elect the new pope.

And he says he wishes to continue to serve the church "through a life dedicated to prayer".