On the morning of 11 September 2001, 19 hijackers took control of four commercial passenger jets flying out of airports on the east coast of the United States.
Two of the aircraft were deliberately flown into the main two towers (the Twin Towers) of the World Trade Center in New York, with a third hitting the Pentagon in Virginia.
The fourth plane never reached its intended target, crashing in Pennsylvania. It is believed that the passengers and crew overpowered the hijackers and took control of the plane.
The Twin Towers were widely considered to be symbols of America's power and influence. The Pentagon is the headquarters of the US Department of Defense.
Both 110-floor World Trade Center towers subsequently collapsed and substantial damage was caused to one wing of the Pentagon. Numerous other buildings at the World Trade Center site in lower Manhattan were destroyed or badly damaged.
The total loss of life on 9/11 was nearly 3,000, including the 19 hijackers. It was the worst loss of life due to a terrorist incident on US soil.
The days that followed saw a significant effect on world economic markets and international confidence.