Ayman al-Zawahiri, an eye surgeon who helped found the Egyptian Islamic Jihad militant group, is often referred to as Osama Bin Laden's right-hand man and the chief ideologue of al-Qaeda.
Zawahiri has become as al-Qaeda's most prominent spokesman
He is believed by some experts to have been the "operational brains" behind the 11 September 2001 attacks in the United States.
Zawahiri was number two - behind only Bin Laden - in the 22 "most wanted terrorists" list announced by the US government in 2001 and continues to have a $25m bounty on his head.
Some experts even suggest Egyptian Islamic Jihad virtually took over al-Qaeda when the two groups forged a coalition in the late 1990s.
Zawahiri was reportedly last seen in the eastern Afghan town of Khost in October 2001, and went into hiding after a US-led coalition overthrew the Taleban.
He and Bin Laden have since evaded capture and are thought to be hiding in the mountainous regions along the Afghanistan-Pakistan border with the help of sympathetic local tribes.
In recent years, Zawahiri has emerged as al-Qaeda's most prominent spokesman, appearing in 16 videos and audiotapes in 2007 - four times as many as Bin Laden - as the group tries to radicalise and recruit Muslims around the world.
Zawahiri's increasingly high profile is thought to have led to a US missile strike on 13 January 2006 near Pakistan's border with Afghanistan aimed at killing him.
Zawahiri's time in prison is said to have profoundly influenced him
The attack killed four al-Qaeda members, but Zawahiri survived and appeared on video two weeks later warning US President George W Bush that neither he nor "all the powers on earth" could bring his death "one second closer".
In July 2007, Zawahiri appeared in a video an hour-and-a-half long, urging Muslims to unite behind al-Qaeda's global jihad and outlining its future strategy.
He said its short-term aim was to attack the interests of the "crusaders and Jews" - the US, its Western allies and Israel.
Its long-term aim is to topple Muslim regimes such as Saudi Arabia and Egypt, and to use Afghanistan, Iraq and Somalia as training grounds for Islamist militants.
In his latest video in December, Zawahiri invited interested individuals, organisations and journalists to submit questions for an "open interview".
Born in the Egyptian capital, Cairo, on 19 June 1951, Zawahiri came from a respectable middle-class family of doctors and scholars.
His grandfather, Rabi'a al-Zawahiri, was the grand imam of al-Azhar, the centre of Islamic learning in the Middle East, while one of his uncles was the first secretary-general of the Arab League.
Zawahiri became involved in political Islam while still at school and was arrested at the age of 15 for being a member of the outlawed Muslim Brotherhood - Egypt's oldest and largest Islamist organisation.
His political activities did not, however, stop him from studying medicine at Cairo University's medical school, from which he graduated in 1974 and obtained a masters degree in surgery four years later.
His father Mohammed, who died in 1995, was a pharmacology professor at the same school.
Zawahiri initially continued the family tradition, building up a prominent medical clinic in a suburb of Cairo, but soon became attracted to radical Islamist groups which were calling for the overthrow of the Egyptian government.
When Egyptian Islamic Jihad was founded in 1973, he joined.
Former Egyptian President Anwar Sadat was killed by Islamic Jihad
In 1981, he was rounded up along with hundreds of other suspected members of the group after several dressed as soldiers assassinated President Anwar Sadat during a military parade in Cairo.
Sadat had angered Islamist activists by signing a peace deal with Israel, and by arresting hundreds of his critics in an earlier security crackdown.
During the mass trial, Zawahiri emerged as a leader of the defendants and was filmed telling the court: "We are Muslims who believe in our religion. We are trying to establish an Islamic state and Islamic society."
Although he was cleared of involvement in Sadat's assassination, Zawahiri was convicted of the illegal possession of arms, and served a three-year sentence.
According to fellow Islamist prisoners, Zawahiri was regularly tortured and beaten by the authorities during his time in jail in Egypt, an experience which is said to have transformed him into a fanatical and violent extremist.
Following his release in 1985, Zawahiri left for Saudi Arabia.
Bin Laden and Zawahiri have been comrades in arms since 1998
Soon afterwards he headed for Peshawar in Pakistan and later to neighbouring Afghanistan, where he established a faction of Egyptian Islamic Jihad whilst working as a doctor in the country during the Soviet occupation.
Zawahiri took over the leadership of Egyptian Islamic Jihad after it re-emerged in 1993, and was a key figure behind a series of attacks by the group on Egyptian government ministers, including the Prime Minister, Atif Sidqi.
The group's campaign to topple the government and set up an Islamic state in the country during the mid-1990s led to the deaths of more than 1,200 Egyptians.
In 1997, the US state department named him as leader of the Vanguards of Conquest group - a faction of Islamic Jihad thought to have been behind the massacre of foreign tourists in Luxor the same year.
Two years later he was sentenced to death in absentia by an Egyptian military court for his role in the group's many attacks.
Zawahiri is thought to have travelled around the world during the 1990s in search of sanctuary and sources of funding.
In the years following the Soviet withdrawal of Afghanistan, he is believed to have lived in Bulgaria, Denmark and Switzerland, and sometimes used a false passport to travel to the Balkans, Austria, Yemen, Iraq, Iran and the Philippines.
In December 1996 he reportedly spent six months in Russian custody after he was caught without a valid visa in Chechnya.
According to an account allegedly written by Zawahiri, the Russian authorities failed to have the Arabic texts found on his computer translated and he was able to keep his identity secret.
In 1997, Zawahiri is believed to have moved to the Afghan city of Jalalabad, where Osama Bin Laden was based.
A year later, Egyptian Islamic Jihad joined five other radical Islamist militant groups, including Bin Laden's al-Qaeda, in forming the World Islamic Front for Jihad against Jews and Crusaders.
The front's first proclamation included a fatwa, or religious edict, permitting the killing of US civilians. Six months later, two simultaneous attacks destroyed the US embassies in Kenya and Tanzania, killing 223 people.
Zawahiri was one of the figures whose satellite telephone conversations were used as proof that Bin Laden and al-Qaeda were behind the plot.
Two weeks after the attacks, the US bombed the group's training camps in Afghanistan. The next day, Zawahiri telephoned a Pakistani journalist and said:
"Tell America that its bombings, its threats, and its acts of aggression do not frighten us. The war has only just begun."