Zawahiri, a joint signatory to Bin Laden's 1998 fatwa calling for attacks on the US, has regularly appeared as spokesman for al-Qa'ida since merging his Islamic Jihad with the terror network in the mid- 1990s.
Known as an astute organiser and military planner, the Egyptian eye surgeon, 54, is credited with being the "mastermind" behind the 11 September 2001 attacks on the US. He has been sentenced in Egypt - in absentia - to death and has a US federal bounty on his head of $25m (£14.4m).
The radical cleric graduated from a bit-part player in Egypt's militant Islamic Gamaa organisation into the leader of Islamic Jihad during the mid-1980s. Several experts believe that Zawahiri is the senior partner in his relationship with the younger Bin Laden.
The scion of an elite Egyptian family of doctors and professionals, he graduated from medical school in Cairo in 1974. He is believed to have been radicalised at medical school, where he had links to the outlawed Muslim Brotherhood and was among a number of suspected militants imprisoned and tortured by Egyptian authorities following the assassination of President Anwar Sadat in 1981.
He spent time in Saudi Arabia, Afghanistan and Pakistan after release and was briefly detained by the Russians for stirring resistance in Dagestan. He is believed to have linked up with the Saudi leader of al-Qa'ida while the pair were living in Khartoum, Sudan. His last confirmed sighting was in Khost, Afghanistan, four years ago, where his wife and children were reportedly killed in a US air strike in November 2001.