The September 11 attacks (often referred to as nine-eleven , written 9/11 ) were a series of coordinated suicide attacks by al-Qaeda upon the United States on September 11, 2001. On that morning, nineteen Islamist terrorists affiliated with al-Qaeda 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 building. Both buildings collapsed within two hours, destroying at least two nearby buildings and damaging others. The hijackers crashed a third airliner into the Pentagon. The fourth plane crashed into a field near Shanksville in rural Somerset County, 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 are no survivors from any of the flights.

Excluding the 19 hijackers, 2,974 people died in the attacks. Another 24 are missing and presumed dead. The overwhelming majority of casualties were civilians, including nationals of over 90 different 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 a War on Terrorism, invading Afghanistan to depose the Taliban, who had harbored al-Qaeda terrorists, and enacting the USA PATRIOT Act. Many other states also strengthened their anti-terrorism legislation and expanded law enforcement powers. Some American stock exchanges stayed closed for the rest of the week, and posted enormous losses upon reopening, especially in the airline and insurance industries. The economy of Lower Manhattan ground to a halt, as billions of dollars in office space was damaged or destroyed.

The damage to the Pentagon was cleared and repaired within a year, and the Pentagon Memorial was built on the site. Rebuilding the World Trade Center site has proven more difficult, with controversy over possible designs as well as the pace of construction. Construction delays, revised cost estimates, security concerns, and public criticism have all led to changes and delays to the final plans in rebuilding the complex to this day.