The September 11 attacks (often referred to as September 11th or 9/11) were a series of coordinated suicide attacks by al-Qaeda upon the United States on September 11, 2001. On that morning, 19 al-Qaeda terrorists hijacked four commercial passenger jet airliners. The hijackers intentionally crashed two of the airliners into the Twin Towers of the World Trade Center in New York City, killing everyone on board and many others working in the buildings. Both buildings collapsed within two hours, destroying nearby buildings and damaging others. The hijackers crashed a third airliner into the Pentagon in Arlington, Virginia, just outside Washington, D.C. The fourth plane crashed into a field near Shanksville in rural Pennsylvania after some of its passengers and flight crew attempted to retake control of the plane, which the hijackers had redirected toward Washington, D.C. There were no survivors from any of the flights.

2,973 victims and the 19 hijackers died as a result of the attacks. The overwhelming majority of casualties were civilians, including nationals of over 70 countries. In addition, the death of at least one person from lung disease was ruled by a medical examiner to be a result of exposure to dust from the World Trade Center's collapse.

The United States responded to the attacks by launching the War on Terrorism. It invaded Afghanistan to depose the Taliban, who had harbored al-Qaeda terrorists. The United States also enacted the USA PATRIOT Act. Many other countries also strengthened their anti-terrorism legislation and expanded law enforcement powers. Some American stock exchanges stayed closed for the rest of the week following the attack and posted enormous losses upon reopening, especially in the airline and insurance industries. The destruction of billions of dollars worth of office space caused serious damage to the economy of Lower Manhattan.

The damage to the Pentagon was cleared and repaired within a year, and the Pentagon Memorial was built on the site. The rebuilding process has started on the World Trade Center site. In 2006 a new office tower was completed on the site of 7 World Trade Center. 1 World Trade Center is currently under construction at the site and, at 1,776 ft (541 m) upon completion in 2013, it will become one of the tallest buildings in North America. Three more towers were originally expected to be built between 2007 and 2012 on the site. Ground was broken for the Flight 93 National Memorial on November 8, 2009, and the first phase of construction is expected to be ready for the 10th anniversary of the attacks on September 11, 2011.