|This undated photo shows al-Zawahiri, left, with bin Laden, centre [AFP]
Ayman al-Zawahiri, who has been announced as the new leader of al-Qaeda, left his life as an Egyptian doctor in the capital Cairo decades ago, in order to dedicate himself to the Islamist underground.
The choice would eventually take him, like Osama bin Laden, the man he has now replaced as head of the group, to the mountains of Afghanistan.
Al-Zawahiri is believed to have been the "operational brains" behind the devastating September 11 2001 attacks on the US, for which bin Laden, who was killed by US Navy SEALs in Pakistan last month, had claimed responsibility.
Al-Zawahiri was born in Cairo in 1951 into a middle-class family of doctors and scholars.
He became involved with political dissent groups early on, and was reportedly arrested as young as 15 for being a member of the then-outlawed Muslim Brotherhood, one of the Arab world's oldest Islamist groups.
In 1985, the 34-year-old, began his life as an underground Islamist, leaving Egypt for Pakistan, where he worked as a doctor treating fighters wounded in battles against Soviet forces occupying neighbouring Afghanistan.
In 1993 he took over the leadership of Jihad, Egypt's second largest Islamic armed group.
A military court in Egypt sentenced al-Zawahiri to death in absentia in 1999 for extremist activities.
He was tried, along with many others, for links to the 1981 assassination of Egyptian President Anwar al-Sadat.
Al-Zawahiri served a three-year jail term for illegal arms possession, but was acquitted of the main charges.
He joined forces with bin Laden in 1998, and was indicted in connection with the bombings that same year of the US embassies in Kenya and Tanzania.
A Cairo-based lawyer, Montasser al-Zayat, said: "Ayman is for bin Laden like the brain to the body."
In December 2001, al-Zawahiri's wife and several children were reported to have been killed by US bombing in Afghanistan, but the al-Qaeda leader managed to avoid the attentions of the US and went on the run.
Enemies of Islam
In a 2003 audiotape, al-Zawahiri urged Muslims to strike at the embassies and commercial interests of the US, United Kingdom, Australia, Norway and half a dozen Middle East states he called subjects of Washington and Israel.
Since then, al-Zawahiri has made numerous statements in videos and audiotapes.
In February 2005, al-Zawahiri condemned the US concept of freedom in a taped speech broadcast by Aljazeera.
The audiotape message said the freedom sought by millions in the Islamic world was "not the freedom to destroy others ... it is not the freedom that allows [America] to support oppressive regimes".
Al-Zawahari has also used taped messages to condemn Tony Blair, the former British prime minister, as well as Queen Elizabeth II and England as "the severest enemies of Islam".
In January 2006, Al Jazeera aired a video in which al-Zawahiri made his first appearance since a US air strike that targeted him in Pakistan.
The air strike reportedly killed four al-Qaeda members. But al-Zawahiri said the raid killed "innocents" as 13 other villagers were also killed in the strike.
He referred to George Bush, the former US president, as "a curse on your own nation," and asked, "Bush, do you know where I am? I am among the Muslim masses."
In November 2008, al-Zawahiri warned Barack Obama, the then president-elect, against sending more troops to Afghanistan and following the policies of his predecessor, Bush.
He reiterated his warnings to the US president in September 2009, saying that Obama's administration would be defeated at the hands of Muslims.
After bin Laden's death in May 2011, al-Zawahiri vowed to press ahead with al-Qaeda's campaign against the US and its allies, in what appeared to be his first public response to bin Laden's death.
"The Sheikh [bin Laden] has departed, may God have mercy on him, to his God as a martyr, and we must continue on his path of jihad to expel the invaders from the land of Muslims and to purify it from injustice," he said in a video message posted online.
"Today, and thanks be to God, America is not facing an individual or a group ... but a rebelling nation which has awoken from its sleep in a jihadist renaissance challenging it wherever it is."
Although al-Zawahiri had long been bin Laden's presumed successor, US officials present him as a far less potent threat.