A trained surgeon from a prominent Egyptian family, Zawahiri is regarded as the brains and ideological force behind al-Qaeda.
Since the Sept. 11, 2001 attacks, Zawahiri has taken an increasingly visible role on behalf of al-Qaeda, appearing regularly in Internet videos, while Osama bin Laden has receded into the shadows, surfacing only occasionally.
That has led to speculation that Zawahiri, 56, has taken over day-to-day operations of al-Qaeda Central and is effectively running the network, although analysts acknowledged its inner working remain opaque, at best.
"Zawahiri is used to dominating from behind the scenes," said Jarret Brachman, research director of the Combating Terrorism Center at the U.S. Military Academy at West Point. "In my opinion, he's sort of like the Dick Cheney of al-Qaeda."
Zawahiri has been involved with subversive movements since he was a teenager and served time in prison for his involvement in the conspiracy to assassinate Egyptian President Anwar Sadat in 1981. He became leader of Egyptian Islamic Jihad, a group dedicated to overthrowing Egypt's secular government, but also presided over its collapse, leaving him many bitter enemies within the radical Islamic movement.
"There is no support or influence for Zawahiri in Egypt," said Osama Rushdi, an Egyptian political exile in Britain who served time with Zawahiri in prison in the early 1980s.
Zawahiri formally merged the remnants of Egyptian Islamic Jihad with bin Laden's al-Qaeda network in 1998, creating a partnership they called the International Islamic Front Against Crusaders and Jews.
Previously, bin Laden's group had been focused on forcing regime change in Afghanistan and Saudi Arabia. But under Zawahiri's influence, al-Qaeda turned its attention to the United States –- resulting in the Aug. 7, 1998, bombings of the U.S. embassies in Kenya and Tanzania.
Zawahiri has been indicted in the attacks by a federal grand jury and the U.S. government has posted a $25 million reward for information on his whereabouts.
Since 2001, Zawahiri has fought ideological battles with other rivals, including Abu Musab al-Zarqawi, the Jordanian fighter who led al-Qaeda's affiliate in Iraq but became notorious for doing things his own way. In a letter recovered by the U.S. military, Zawahiri chastised Zarqawi for many of his tactics, including his gruesome decapitation videos and attacks on Shiite civilians.
Zarqawi was killed in a U.S. air strike in June 2006, but Zawahiri hasn't lost his penchant for reacting to perceived slights.
On May 5, in a videotaped speech, Zarqawi included in his rambling script a detailed personal rebuttal to a paper written by Brachman and a West Point colleague in which they argued that the U.S. government should exploit divisions in radical Islamic circles by financing a cleric who is one of Zawahiri's ideological rivals.
Brachman said he was a little shocked that Zawahiri had read the paper, which had circulated on the Internet. "They plastered my name and our logo all over," he said. "It got under their skin."
— Craig Whitlock