Where and when? 9 p.m. ET at Hofstra University, Long Island, New York.
Who is moderating? CNN's Candy Crowley will moderate a presidential debate for the first time. She is the first female presidential debate moderator in 20 years.
What is the format? This debate will be "town-hall" style. Undecided voters selected by Gallup will pose questions to the candidates. Both candidate will have two minutes to respond to each question. Crowley said Monday night that she plans to ask the candidates follow-up questions, despite both campaigns' claims that doing so was outside the debate rules. Those rules were agreed to by the campaigns, but not by Crowley.
Where can I watch it? You can watch the debate on your computer or mobile device on The Washington Post Grid, and most broadcast networks and cable news outlets are airing it in its entirety.
How can I follow it on social media? The official Twitter handle is #debates. Tumblr will again live-GIF the debate in conjunction with the Guardian here. Each candidate has a page dedicated to the event -- here is the Romney debate page and the Obama debate page.