Fourteen years ago, on the 11th September 2001, nearly 3,000 people were killed in a terrorist attack in the USA.
Extremists hijacked four passenger jets.
They used two of them to destroy the Twin Towers skyscrapers of the World Trade Center, in Manhattan, downtown New York.
A third plane crashed into the Pentagon, America's military headquarters in Washington D.C.
A fourth plane went down in fields near Pittsburgh.
As thousands of workers scrambled to escape the burning buildings, hundreds of rescue workers rushed to get in.
The south tower collapsed first and then the north tower.
Around 300 firefighters were among those killed.
The group behind the attack was Al Qaeda, an islamic terrorist organization.
America, backed by the UK and other countries, promised to strike back.
They launched the Afghan conflict (in October 2001); in Afghanistan the ruling Taliban supported the organisation.
They also launched the second Iraq War (in 2003); it has since been determined that Iraq was not linked to Al Qaeda and many think the invasion of Iraq made things worse there, not better.
In May 2011, American troops tracked down and killed Osama bin Laden, the leader of Al Qaeda.
Terror suspects remain locked up without trial in Guantanamo, a controversial US detention centre in Cuba.
The 9/11 Memorial to the victims was opened in 2011.
Click HERE to watch a video commemorating 9/11.