Ayman al-Zawahri was appointed the new leader of al-Qaeda, the notorious Islamist terrorist group responsible for the attacks of Sept. 11, 2001, in June 2011 following the death of Osama bin Laden at the hands of U.S. special forces.
Like his predecessor, the Egyptian-born al-Zawahri comes from a well-connected and wealthy family. He trained as a surgeon but has long acted as al-Qaeda's No. 2, providing tactical and organizational skills and promoting the use of suicide bombings and the independent cells that have become the network's trademarks.
Fluent in English, French and Arabic, al-Zawahri has been the chief ideologue of the group, and many suspect he was the operational brains behind the 9/11 attacks.
Al-Zawahri has appeared in dozens of al-Qaeda videos and audio recordings in recent years, increasingly becoming the group's public face as bin Laden sought to keep a lower profile.
Early activism in Egypt
Al-Zawahri was born on June 19, 1951, and raised in an affluent Cairo neighbourhood.
His father was a prominent physician, and many of his relatives held important positions in Egypt's political and religious circles.
He graduated from Cairo University's medical school in 1974, eventually earning a master's degree in surgery in 1978.
Although accounts differ, al-Zawahri apparently became involved with the Islamist fundamentalist movement in Egypt at a young age, possibly as early as his teens.
By the late 1970s, he was an active member of Egyptian Islamic Jihad, an outlawed organization that sought to overthrow the government and was responsible for the assassination of Egyptian president Anwar Sadat in 1981.
Al-Zawahri was among hundreds of the group's members arrested in connection with Sadat's death. He was sentenced to three years in jail on weapons charges but acquitted of involvement in the assassination.
Meets bin Laden in Afghanistan
According to some reports, al-Zawahri emerged from prison in 1984 as one of the top Islamist militants in Egypt, becoming something of a spokesman for the fundamentalist movement.
One year later, he travelled to Afghanistan and Pakistan, taking part in the jihad against the Soviet invasion of the country. Most accounts say he worked primarily as a surgeon, treating injured mujahedeen fighters.
It was during this time that al-Zawahri first met bin Laden, with whom he became close.
Following the Soviet withdrawal from Afghanistan in 1989, he continued to rise in the ranks of Egyptian Islamic Jihad and became leader of the group four years later.
Around this time, al-Zawahri travelled to the United States and Europe to raise funds for the group. In 1996, he was detained in Russia for six months after attempting to recruit jihadists in Chechnya.
Egyptian Islamic Jihad and al-Qaeda merged in 1998, and al-Zawahri became second in command to bin Laden.
He was indicted that same year by the U.S. government for his alleged role in the bombing of U.S. embassies in Kenya and Tanzania that killed more than 200 people and injured thousands.
$25M US bounty
Following the attacks on the U.S. in 2001, in which about 3,000 people died, al-Zawahri was added to the FBI's list of most-wanted terrorists.
He went into hiding after the U.S.-led invasion of Afghanistan that followed the 9/11 attacks. His wife and several of his children were reportedly killed in an air strike by U.S. and allied forces near Tora Bora, Afghanistan, in December 2001.
Following the death of bin Laden, al-Zawahri released a videotaped eulogy for the al-Qaeda leader in which he warned of an uprising by an international community of Muslims.
"Today, praise God, America is not facing an individual, a group or a faction," he said. "It is facing a nation that is in revolt, having risen from its lethargy to a renaissance of jihad."
The U.S. government is currently offering up to $25 million US for information leading to the arrest or conviction of al-Zawahri, whom many suspect is hiding somewhere near the Afghanistan-Pakistan border.