The September 11 attacks (also referred to as 9/11)were a series of four coordinated terrorist attacks by the Islamic terrorist group Al-Qaeda on the United States on the morning of Tuesday, September 11, 2001. The attacks consisted of suicide attacks used to target symbolic U.S. landmarks.

Four passenger airliners—which all departed from airports on the U.S. East Coast bound for California—were hijacked by 19 al-Qaeda terrorists to be flown into buildings. Two of the 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 an hour and 42 minutes, both 110-story towers collapsed, with debris and the resulting fires causing partial or complete collapse of all other buildings in the World Trade Center complex, including the 47-story 7 World Trade Center tower, 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) in Arlington County, Virginia, leading to a partial collapse in the Pentagon's western side. The fourth plane, United Airlines Flight 93, initially was steered toward Washington, D.C., but crashed into a field near Shanksville, Pennsylvania, after its passengers tried to overcome the hijackers. In total, the attacks claimed the lives of 2,996 people (including the 19 hijackers) and caused at least $10 billion in property and infrastructure damage and $3 trillion in total costs. It was the deadliest incident for firefighters and law enforcement officers in the history of the United States, with 343 and 72 killed respectively.

Suspicion for the attack quickly fell on Al-Qaeda. 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 the powers of law enforcement and intelligence agencies to prevent terrorist attacks. Although al-Qaeda'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. Having evaded capture for almost a decade, bin Laden was located and killed by members of the U.S. military in May 2011.

The destruction of the World Trade Center and nearby infrastructure 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, out of respect or fear of further attacks. Cleanup of the World Trade Center site was completed in May 2002, and the Pentagon was repaired within a year. On November 18, 2006, construction of One World Trade Center began at the World Trade Center site. The building was officially opened on November 3, 2014.Numerous memorials have been constructed, including the National September 11 Memorial & Museum in New York City, the Pentagon Memorial in Arlington County, Virginia, and the Flight 93 National Memorial in a field near Shanksville, Pennsylvania.