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.
Al-Zawahiri is described as the "mastermind" behind al-Qaeda attacks
He is believed by some experts to have been the "operational brains" behind the 11 September 2001 attacks in the United States.
He was number two - behind only Bin Laden - in the 22 Most Wanted Terrorists List announced by the US government in 2001 and has a $25m bounty on his head.
Some experts even suggest Zawahiri's Egyptian Islamic Jihad virtually took over al-Qaeda, when the two groups forged a coalition in the late 1990s.
He was reportedly last seen in the eastern Afghan town of Khost in October 2001, and went into hiding after the US-led attack overthrew the Taleban.
Various sources have said he may have escaped to North Africa or the Middle East, but US officials believe he is still hiding in the Afghan-Pakistan border area.
Often seen beside Bin Laden in videos released to Arab television networks, Zawahiri was also thought to have served as the al-Qaeda leader's personal physician.
Bin Laden and Zawahiri are comrades in arms
CIA officials said an audio tape aired on Arab television station in March 2004 - urging Pakistanis to overthrow President Pervez Musharraf - was probably the voice of Zawahiri.
It came days after a major operation against al-Qaeda militants was launched by Pakistani troops in the South Waziristan tribal area, on the Afghan border.
Initial speculation was that Zawahiri had been among al-Qaeda leaders surrounded by thousands of soldiers but he was never captured, and the army later conceded that many suspects may have escaped through a network of secret tunnels.
Giles Foden, author of a book on the 1998 US embassy bombings in Kenya and Tanzania, says even labelling Zawahiri as Bin Laden's right-hand man may understate his importance.
He says some analysts believe Zawahiri has been controlling much of al-Qaeda's finance operations since the end of the war in Afghanistan.
Zawahiri is named in European legislation on financial sanctions against the Taleban and in documents produced by the US sanctions body, the US Treasury's office for foreign assets control.
Born in Egypt in 1951, Ayman al-Zawahiri, comes from a middle class family of doctors and scholars.
His grandfather, Rabi'a al-Zawahiri, was the grand imam of Cairo's al-Azhar university, a centre of Islamic learning in the Arab world.
He was already involved in Egypt's radical Muslim community when he was arrested at the age of 15 for being a member of the outlawed Muslim Brotherhood - the Arab world's oldest fundamentalist group.
He graduated from Cairo University's medical school in 1974 and obtained a masters degree in surgery four years later.
His father, who died in 1995, was a pharmacology professor at the same school.
Zawahiri's wife and children were reportedly killed in a US air strike in Afghanistan in late November or early December 2001.
Zawahiri was tried along with scores of radical Islamists for their part in the 1981 assassination of President Anwar Sadat during a Cairo military parade.
He was convicted and served a three-year sentence for illegal possession of arms. After his release, he left for Saudi Arabia.
Soon afterwards he headed for Peshawar and later to neighbouring Afghanistan, where he established a faction of the Egyptian Islamic Jihad group.
Zawahiri was in Afghanistan during the Soviet occupation in 1970s and 1980s, working as a doctor.
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 court for activities linked to Islamic Jihad.
Zawahiri reportedly spent six months in the Russian custody for his alleged extremist activities in the southern republic of Dagestan.
But he was released as his name was not known to Russia's secret services.
Zawahiri is believed to have lived in Denmark and Switzerland in the early 1990s, sometimes travelling on a false passport.
Giles Foden says Zawahiri's freewheeling role across western Europe during the early 1990s raises questions about the security and asylum policies of a number of European nations and about their refusal to act on information provided by the Egyptian government.
Zawahiri appeared in a video alongside Bin Laden threatening retaliation against the United States for the detention of the Egyptian Sheikh Omar Abdel Rahman in connection with the 1993 World Trade Center bombing.
Then, in 1998, he was the second of five signatories to Bin Laden's notorious 1998 "fatwa" calling for attacks against US civilians.
He is also listed on the US government's indictment sheet for the 1998 US embassy bombings.
He was one of the figures whose satellite telephone conversations were used as proof that Bin Laden was behind the plot.