Events are being held around the world in memory of the almost three thousand people who died when terrorists hijacked passenger planes and flew them into the Pentagon and the Twin Towers in New York.
Joe Biden will mark the anniversary by visiting each of the sites where the planes crashed in 2001.
An Irish former NYPD officer involved in the recovery mission at Ground Zero says 'all hell broke loose' when the terrorists struck.
Paul McCormack spent several months working in the wreckage... which resulted in the serious deterioration of his eyesight.
He's been in Dallas this week for a ceremony to mark the attacks:
Bertie Ahern was Taoiseach when 9/11 happened 20 years ago today:
First responder Will Jimeno was pulled alive from the rubble of the World Trade Center.
He still can't believe he made it: