President Barack Obama and GOP nominee Mitt Romney square off tonight at 9 p.m. ET at Hofstra University in Hempstead, N.Y. For a preview of what's in store, check out our viewer's guide, or go back in time for a comprehensive history of presidential debates. There has been much controversy surrounding this debate's town hall format, with both sides reportedly concerned about how active a role moderator Candy Crowley will play in helping to steer questions from undecided voters, this year's most elusive demographic. And don't miss: 5 ways Mitt Romney can have another strong debate performance, and 5 ways Obama can bounce back and win this one.