Egyptian-born doctor and surgeon Ayman al-Zawahiri succeeded as al Qaeda's succeeding Osama bin Laden, who was killed by US special forces in Pakistan last month.
Following are some key facts about Zawahiri:
* Zawahiri is described as the chief organizer of al Qaeda and was bin Laden's closest mentor.
* Zawahiri and bin Laden met in the mid-1980s when both were in the Pakistani city of Peshawar to support mujahideen guerrillas fighting the Soviets in Afghanistan.
* Born in 1951 to a prominent Cairo family, Zawahiri was the son of a pharmacology professor and grandson of the grand imam of Al Azhar, one of the most important mosques in the Arab world.
* He graduated from Egypt's most prestigious medical school in 1974.
* When the militant Egyptian Islamic Jihad was founded in 1973, he joined. When members posed as soldiers and assassinated President Anwar Sadat in 1981, he was among 301 people arrested.
He went on trial but was cleared of involvement in Sadat's killing. He did, however, spend three years in jail for possession of an unlicensed pistol.
* Zawahiri has broadcast dozens of messages since the Sept. 11 attacks on the United States in 2001. In April, he urged Muslims to fight Nato and American forces in Libya.
* In Jan. 2006 Zawahiri called US President George W Bush a "butcher" in a video tape, saying a US air strike targeting him had killed only innocent people.
Pakistani intelligence sources said four top al Qaeda militants were believed to be killed in a US air strike, which US officials say was aimed at Zawahiri.
* Earlier this month Zawahiri released a new video recording following the death of bin Laden in May, vowing to press ahead with the group's campaign against the United States and its allies.