For two decades al Qaeda, whose leader Osama bin Laden was killed on Monday, has been linked to dozens of atrocities around the world, but it was the 9/11 attacks on the US that shot the group to infamy.
Believed to be more often a brand used by disparate groups than a structured organisation, al Qaeda simply means "the base" in Arabic.
Bin Laden was seen by his supporters as the inspiration rather than the organiser of attacks. His number-two, Ayman al-Zawahiri, is considered the real mastermind and intellectual heavyweight.
The organisation first surfaced around 1988, in the final years of the guerrilla war waged by a variety of Islamic groups against Soviet troops in Afghanistan.
Al Qaeda figurehead bin Laden, from a Saudi family which built its wealth in construction, helped train anti-Soviet militants in the mountains along the border between Afghanistan and Pakistan.
After 1989, when the Soviets left, bin Laden moved back to his native Saudi Arabia, but he believed his calling had only just begun.
When Saudi Arabia, the guardians of Islam's holiest places, allowed the United States to deploy troops on their territory during the 1991 Gulf War, it was the supreme insult for a fundamentalist such as bin Laden.
Al Qaeda went on to claim attacks on US forces in Saudi Arabia, Yemen and Somalia, but it was not until around 1998 that it obtained, albeit briefly, a territorial base.
Bin Laden and his fellow activists were welcomed to Afghanistan by the austere Taliban movement, which had fought its way to power in 1996.
Two years later bin Laden announced that he had formed a front "to carry out a holy war against the Crusaders and Jews," targeting Americans, civilians and US troops.
He was soon to turn those threats into action: in August of the same year twin bomb attacks on the US embassies in Nairobi and Dar es Salaam killed 224 people.
Despite the increasing deadliness of attacks claimed by al Qaeda, the United States was stunned on September 11, 2001, when hijacked airliners brought down the World Trade Center towers in New York and punched a hole in the Pentagon.
The attacks illustrated a particularly potent aspect of al Qaeda: the willingness of its operatives to die for their cause.
Nineteen hijackers were among some 3,000 people to die in the attacks.
Then US president George W Bush declared a "war on terror", initially against al Qaeda's bases in Afghanistan and then against Iraq, a country which had no connection with the events of 2001.
The invasion of Iraq led to a particularly deadly insurgent group branding itself to al Qaeda -- al Qaeda in the Land of the Two Rivers.
At the height of its activities, in 2005 and 2006, many of its worst attacks were against Shiite Muslims -- seen as apostates by extremist Sunnis such as bin Laden -- rather than US forces.
Another group using the al Qaeda label, in north Africa, is mainly based in Algeria. It has been seeking since 2006 to bring under its wing armed movements in Tunisia, Morocco and the Sahel.
In bin Laden's ancestral homeland of Yemen, Saudi and Yemeni al Qaeda branches in January 2009 announced they had merged to form the Yemen-based al Qaeda in the Arabian Peninsula (AQAP).
But whatever the reality behind the label, the al Qaeda name has been linked to attacks all over the world, from Madrid in 2004 to London in 2005.