The World Trade Center experienced a fire on February 13, 1975, a bombing on February 26, 1993, and a robbery on January 14, 1998 In 1998, the Port Authority decided to privatize the World Trade Center, leasing the buildings to a private company to manage, and awarded the lease to Silverstein Properties in July 2001.

 On the morning of September 11, 2001, Al-Qaeda-affiliated hijackers flew two Boeing 767 jets into the North and South Towers within minutes of each other; two hours later, both had collapsed. The attacks killed 2,606 people in and within the vicinity of the towers, as well as all 157 on board the two aircraft. Falling debris from the towers, combined with fires that the debris initiated in several surrounding buildings, led to the partial or complete collapse of all the buildings in the complex and caused catastrophic damage to ten other large structures in the surrounding area. The cleanup and recovery process at the World Trade Center site took eight months, during which the remains of the other buildings were demolished.

 The World Trade Center complex was rebuilt over more than a decade. The site is being rebuilt with six new skyscrapers, while a memorial to those killed in the attacks and a new rapid transit hub have both opened. One World Trade Center, the tallest building in the United States, is the lead building for the new complex, reaching more than 100 stories on completion in November 2014.

Four passenger airliners operated by two major U.S. passenger air carriers (United Airlines and American Airlines) all of which departed from airports in the northeastern United States bound for California — were hijacked by 19 al-Qaeda terrorists. Two of the planes, American Airlines Flight 11and 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 of the building's western side. The fourth plane, United Airlines Flight 93, was initially steered toward Washington, D.C., but crashed into a field in Stonycreek Township near Shanksville, Pennsylvania, after its passengers tried to overcome the hijackers. 9/11 was the single deadliest incident for firefighters and law enforcement officers in the history of the United States, with 343 and 72 killed respectively.

 Suspicion quickly fell on al-Qaeda. The United States responded 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. After evading capture for almost a decade, Osama bin Laden was located and killed by SEAL Team Six of the U.S. Navy 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, resulting in the closing of 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 memorialshave 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 in Stonycreek Township near Shanksville, Pennsylvania.

The damaged section of the Pentagon was rebuilt and occupied within a year of the attacks. The temporary World Trade Center PATH station opened in late 2003 and construction of the new 7 World Trade Center was completed in 2006. Work on rebuilding the main World Trade Center site was delayed until late 2006 when leaseholder Larry Silverstein and the Port Authority of New York and New Jersey agreed on financing.The construction of One World Trade Center began on April 27, 2006, and reached its full height on May 20, 2013. The spire was installed atop the building at that date, putting 1 WTC's height at 1,776 feet (541 m) and thus claiming the title of the tallest building in the Western Hemisphere.One WTC finished construction and opened on November 3, 2014.

On the World Trade Center site, three more office towers are expected to be built one block east of where the original towers stood. Construction has begun on all three of these towers.

One of the first memorials was the Tribute in Light, an installation of 88 searchlights at the footprints of the World Trade Center towers. In New York City, the World Trade Center Site Memorial Competition was held to design an appropriate memorial on the site.The winning design, Reflecting Absence, was selected in August 2006, and consists of a pair of reflecting pools in the footprints of the towers, surrounded by a list of the victims' names in an underground memorial space. The memorial was completed on September 11, 2011; a museum also opened on site on May 21, 2014.

In Arlington County, The Pentagon Memorial was completed and opened to the public on the seventh anniversary of the attacks in 2008. It consists of a landscaped park with 184 benches facing the Pentagon. When the Pentagon was repaired in 2001–2002, a private chapel and indoor memorial were included, located at the spot where Flight 77 crashed into the building.

 In Shanksville, a concrete and glass visitor center was opened on September 10, 2015, situated on a hill overlooking the crash site and the white marble Wall of Names. An observation platform at the visitor center and the white marble wall are both aligned beneath the path of Flight 93. A temporary memorial is located 500 yards (457 m) from the crash site. New York City firefighters donated a cross made of steel from the World Trade Center and mounted on top of a platform shaped like the Pentagon. It was installed outside the firehouse on August 25, 2008. Many other permanent memorials are elsewhere. Scholarships and charities have been established by the victims' families, and by many other organizations and private figures.

On every anniversary, in New York City, the names of the victims who died there are read out against a background of somber music. The President of the United States attends a memorial service at the Pentagon,and asks Americans to observe Patriot Day with a moment of silence. Smaller services are held in Shanksville, Pennsylvania, which are usually attended by the President's spouse.