The September 11 attacks (also referred to as September 11, September 11th, or 9/11) were a series of four coordinated terrorist attacks launched by the Islamic terrorist group al-Qaeda upon the United States in New York City and the Washington, D.C., metropolitan area on Tuesday, September 11, 2001. The attacks killed almost 3,000 people and caused at least $10 billion in property and infrastructure damage.

Four passenger airliners were hijacked by 19 al-Qaeda terrorists so they could be flown into buildings in suicide attacks. Two of those planes, American Airlines Flight 11 and United Airlines Flight 175, were crashed into the North and South towers, respectively, of the World Trade Center complex in New York City. Within two hours, both towers collapsed with debris and the resulting fires causing partial or complete collapse of all other buildings in the WTC complex, as well as significant damage to ten other large surrounding structures. A third plane, American Airlines Flight 77, was crashed into the Pentagon (the headquarters of the United States Department of Defense), leading to a partial collapse in its western side. The fourth plane, United Airlines Flight 93, was targeted at Washington, D.C. but crashed into a field near Shanksville, Pennsylvania, after its passengers tried to overcome the hijackers. In total, almost 3,000 people died in the attacks, including the 227 civilians and 19 hijackers aboard the four planes. It also was the deadliest incident for firefighters and for law enforcement officers in the history of the United States, with 343 and 72 killed respectively.

Suspicion quickly fell on al-Qaeda. Although the group's leader, Osama bin Laden, initially denied any involvement, in 2004, he claimed responsibility for the attacks.

Al-Qaeda and bin Laden cited U.S. support of Israel, the presence of U.S. troops in Saudi Arabia, and sanctions against Iraq as motives for the attacks. The United States responded to the attacks by launching the War on Terror and invading Afghanistan to depose the Taliban, which had harbored al-Qaeda. Many countries strengthened their anti-terrorism legislation and expanded law enforcement powers. Having evaded capture for years, bin Laden was located and killed by U.S. forces in May 2011.

The destruction of the Twin Towers and other properties caused serious damage to the economy of Lower Manhattan and had a significant effect on global markets, closing Wall Street until September 17 and the civilian airspace in the U.S. and Canada until September 13.

Many closings, evacuations, and cancellations followed the attack, either out of fear of further attacks or respect for the tragedy. Cleanup of the World Trade Center site was completed in May 2002, and the Pentagon was repaired within a year. Numerous memorials have been constructed, including the National September 11 Memorial & Museum in New York, the Pentagon Memorial, and the Flight 93 National Memorial in Pennsylvania.

On November 18, 2006, construction of One World Trade Center began at the World Trade Center site.

As of September 2013, the new tower's concrete construction was largely complete, and will officially open when the installation of podium glass and interior construction is completed, estimated 2014