Ayman al-Zawahiri, an eye surgeon who helped found the Egyptian Islamic Jihad militant group, is expected to replace Osama Bin Laden as the leader of al-Qaeda.
He was already the group's chief ideologue and was believed by some experts to have been the "operational brains" behind the 11 September 2001 attacks
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 25 million dollar bounty on his head.
Zawahiri was reportedly last seen in the eastern Afghan town of Khost in October 2001, and went into hiding after the overthrow of the Taliban. He was thought to be hiding in the mountainous regions along the Afghanistan-Pakistan border with the help of sympathetic local tribes. However, the killing of Bin Laden on 1 May 2011 in Abbottabad, north of the Pakistani capital Islamabad, suggests this may not be the case. His wife and children were reportedly killed in a US air strike in late 2001. Zawahiri was for a time al-Qaeda's most prominent spokesman, appearing in 40 videos and audiotapes since 2003 - most recently in April 2011 - as the group tried to radicalize and recruit Muslims worldwide. He has also been indicted in the US for his role in the 1998 US embassy bombings in Africa, and was sentenced to death in Egypt in absentia for his activities with Islamic Jihad during the 1990s.
Abu Yahya al-Libi, also known as Hasan Qayid and Yunis al-Sahrawi, is thought to have been a member of the Libyan Islamic Fighting Group (LIFG) before he allied himself to Osama Bin Laden. He has since emerged as al-Qaeda's leading theologian, and most visible face on video, surpassing Ayman al-Zawahri in recent years.
Libi is believed to have spent five years as a religious student in Mauritania in the 1990s.
He claims he was captured by Pakistani forces in 2002 and then sent to the US military airbase at Bagram in Afghanistan, from where he escaped in July 2005 along with three other al-Qaeda members (this is probably true but there are no records of it).
Al-Qaeda has named Libi as a field commander in Afghanistan, though he has styled himself in his many videos as a theological scholar, and spoken on a variety of global issues of importance to the group.
Khalid al-Habib, thought to be either Egyptian or Moroccan, was identified in a November 2005 video as al-Qaeda's field commander in south-east Afghanistan, while Abd al-Hadi al-Iraqi was named as its commander in the south-west.
Habib seems to have assumed overall command after the latter's capture in 2006.
He was described as al-Qaeda's "military commander" in July 2008. US military officials say he oversees al-Qaeda's "internal" operations in Afghanistan and northern Pakistan.
Habib may be operating under an assumed identity, according to some analysts. One of his noms de guerre is believed to be Khalid al-Harbi.
In August 2010, the FBI said Adnan Gulshair el Shukrijumah had taken over as chief of al-Qaeda's "external operations council". Having lived for more than 15 years in the US, it is the first time a leader intimately familiar with American society has been placed in charge of planning attacks for the group outside Afghanistan. Such a position - once held by the alleged mastermind of 9/11, Khalid Sheikh Mohammed - necessitates regular contact with al-Qaeda's senior leadership and military commanders, and makes him likely to be killed or captured.
Born in Saudi Arabia, Shukrijumah moved to the US when his father, a Muslim cleric, took up a post at a mosque in Brooklyn. They later moved to Florida (what is it about Florida with these guys?).
In the late 1990s, he became convinced that he had to participate in jihad in place like Chechnya, and left for training camps in Afghanistan. Shukrijumah has been named in a US federal indictment as a conspirator in the case against three men accused of plotting suicide bomb attacks on New York's subway system in 2009. He is also suspected of having played a role.
A Libyan, Atiyah Abd al-Rahman joined Osama Bin Laden in Afghanistan as a teenager in the 1980s. Since then, he has gained considerable stature in al-Qaeda as an explosives expert and Islamic scholar. He retreated with Bin Laden to the mountainous Afghanistan-Pakistan border region in late 2001, and has since become a link to other Islamist militant groups in the Middle East and North Africa.
In June 2006 the US military recovered a letter he wrote to the late Abu Musab al-Zarqawi, the Jordanian who ran al-Qaeda in Iraq, chastising him for alienating rival insurgent groups and attacking Shia Muslims. It warned Zarqawi that he could be replaced if he did not change his ways.
He is said to have successfully brokered a formal alliance with the Algerian Salafist Group for Preaching and Combat (GSPC), which changed its name to al-Qaeda in the Islamic Maghreb.
An Egyptian in his late 30s, Saif al-Adel is the nom de guerre of a former Egyptian army colonel, Muhamad Ibrahim Makkawi. He travelled to Afghanistan in the 1980s to fight Soviet forces with the mujahideen.
Adel was once Osama Bin Laden's security chief, and assumed many of military commander Mohammed Atef's duties after his death in a US air strike in November 2001.
He is suspected of involvement in the 1998 US embassy bombings in East Africa, training the Somali fighters who killed 18 US servicemen in Mogadishu in 1993, and instructing some of the 11 September 2001 hijackers.
In 1987, Egypt accused Adel of trying to establish a military wing of the militant Islamic group al-Jihad, and of trying to overthrow the government. Following the invasion of Afghanistan, Adel is believed to have fled to Iran with Suleiman Abu Ghaith and Saad Bin Laden, a son of the late al-Qaeda leader. They were allegedly then held under house arrest by the Iran's Islamic Revolutionary Guard Corps (IRGC). Iran has never acknowledged their presence.
Several letters and internet statements bearing Adel's name or aliases have been released since 2002, leading analysts to believe he is still in contact with al-Qaeda's leaders in the region. Recent reports say Adel may have been released and made his way to northern Pakistan, along with Saad Bin Laden.
A radical American Muslim cleric of Yemeni descent, Awlaki has been linked to a series of attacks and plots across the world - from 11 September 2001 to the shootings at Fort Hood in November 2009.
Since going on the run in Yemen in December 2007, Awlaki's overt endorsement of violence as a religious duty in his sermons and on the internet is thought to have inspired new recruits to Islamist militancy. US officials say he is also a leader of al-Qaeda in the Arabian Peninsula, an offshoot of the militant network in Saudi Arabia and Yemen, and helped recruit Umar Farouk Abdulmutallab, the Nigerian accused of attempting to blow up an airliner as it flew into Detroit on 25 December 2009.
Following the failed attack, President Barack Obama took the extraordinary step of authorising the Central Intelligence Agency to kill him. Soon afterwards, Awlaki survived an air strike in southern Yemen.
Awlaki is currently thought to be hiding in the mountainous governorates of Shabwa and Marib, under the protection of the large and powerful Awalik tribe, to which he belongs. His family say he is not a terrorist.
Live Long and Prosper....