First of all, wow! His “My Fellow Americans” letter may be found here. The key paragraph:

It has been the greatest honor of my life to serve as your President. And while it has been my intention to seek reelection, I believe it is in the best interest of my party and the country for me to stand down and to focus solely on fulfilling my duties as President for the remainder of my term. 

Democrats now have to choose a nominee, and we’re in uncharted territory (at least in the modern era). The Washington Post has this graphic explanation of what could happen next this morning, Jeff Greenfield this piece in Politico yesterday, and Vox this one just a few minutes ago. More to come, I’m sure . . .

Update: Biden has issued a second statement, endorsing VP Harris to be the party’s nominee.