Almost 3,000 people lost their lives, and many more were injured in the terrorist attacks that struck America.
With the tenth anniversary of that terrible day approaching, tributes are being held across the US.
This weekend in New York thousands will hold hands along the waterfront of Lower Manhattan, forming a human chain heading north, to "reflect, remember and unite."
And on the day of the anniversary the National 9/11 Memorial at the former World Trade Center complex will open for a special dedication ceremony for families of the victims.
We take a look at the changing aspect of 'Ground Zero' since that fateful Tuesday morning ten years ago.