Al-Qaeda has evolved as prominent figures are killed - including, of course, its leader Osama Bin Laden in 2011 - and the geographical focus of militant activity shifts.
Here we profile some of the most prominent names:Ayman al-Zawahiri
Ayman al-Zawahiri (16 December 2007)Ayman al-Zawahiri, an eye surgeon who helped found the Egyptian militant group Islamic Jihad, was named as the new leader of al-Qaeda on 16 June 2011, a few weeks after Osama Bin Laden's death.
In a statement, al-Qaeda vowed to continue its jihad under the new leadership against "crusader America and its servant Israel, and whoever supports them".
Zawahiri 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 in the US.
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 $25m (?16m) bounty on his head.
One of his wives and two of their children were killed in a US air strike in late 2001.
Zawahiri went into hiding after a US-led coalition overthrew the Taliban. Security analysts believe he is most likely to be concealed in the Afghan-Pakistan border region, although he has continued to evade capture and his precise whereabouts are unknown.
In January 2006, the US launched an airstrike on Damadola, a Pakistani village near the Afghan border where they believed Zawahiri was hiding, killing 18 villagers including four children.
US sources suggested he was among the dead in international media over the following days - only for a video to be released showing that he was unharmed.
Zawahiri has been one of al-Qaeda's most prominent spokesman, appearing in dozens of videos and audiotapes since 2003 - most recently in September 2014, when he called for an Islamist resurgence in India.
He has 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.
In June 2013 Zawahiri called for the radical Islamist group Islamic State (IS) to leave Syria and instead focus on Iraq, and in February 2014 al-Qaeda severed all ties with the group altogether.Nasser Abdul Karim al-Wuhayshi
Wuhayshi, a former private secretary to Osama Bin Laden, is the leader of al-Qaeda in the Arabian Peninsula (AQAP), which was formed in 2009 in a merger between two offshoots of al-Qaeda in Saudi Arabia and Yemen.
US news organisation CNN quotes a US official as saying intelligence suggests he has recently been appointed as al-Qaeda's second-in-command - its "general manager" - by Ayman al-Zawahiri, adding weight to claims that al-Qaeda is reorienting from the Afghanistan-Pakistan region to the Arab world. He is said to be only 36 years old.
Wuhayshi replaces "the Libyan", Abu Yahya al-Libi, killed by a US drone strike in Pakistan's north-west in June 2012.
Nasser Abdul Karim al-Wuhayshi
US counter-terrorism officials have called AQAP the "most active operation franchise" of al-Qaeda beyond Pakistan and Afghanistan.
Wuhayshi, who is from the southern Yemeni governorate of al-Baida, spent time in religious institutions before travelling to Afghanistan in the late 1990s.
He fought at the battle of Tora Bora in December 2001, before escaping over the border into Iran, where he was eventually arrested. He was extradited to Yemen in 2003.
In February 2006, Wuhayshi and 22 other suspected al-Qaeda members managed to escape from a prison in Sanaa. Among them were also Jamal al-Badawi, the alleged mastermind of the USS Cole bombing, and Qasim al-Raymi, AQAP's military commander.
After their escape from prison, Wuhayshi and Raymi are said to have overseen the formation of al-Qaeda in Yemen, which took in both new recruits and Arab fighters returning from Iraq and Afghanistan.
The group claimed responsibility for two suicide bomb attacks that killed six Western tourists before being linked to the assault on the US embassy in Sanaa in 2008, in which 10 Yemeni guards and four civilians died.
Four months later, Wuhayshi announced in a video the merger of the al-Qaeda offshoots in Yemen and Saudi Arabia to form "al-Qaeda of Jihad Organisation in the Arabian Peninsula" and his appointment as AQAP leader was later confirmed by Zawahiri.
The group's first operation outside Yemen was carried out in Saudi Arabia in August 2009 against the kingdom's security chief, Prince Mohammed bin Nayef, though he survived.
It later said it was behind the attempt to blow up a US passenger jet as it flew into Detroit on 25 December 2009. A Nigerian man charged in relation with the incident said AQAP operatives had trained him.
Two more plots targeting US aviation were foiled.
At home, Wuhayshi's group capitalised on Yemen's political turmoil to capture large regions of territory in 2011, only to be driven out of many areas in an army offensive in 2012. In recent months, it has been blamed for a growing number of bombings targeting Yemeni security services - mirroring US drone strikes which analysts say nearly tripled in Yemen in 2012 from 2011.
On 10 September 2012, officials in Yemen said Wuhayshi's deputy, Saudi-born Said al-Shihri, had been killed in an air strike in Hadramawt in southern Yemen. The group has vowed to avenge that and other killings of senior AQAP figures.Khalid al-Habib
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.
In early 2006, Pakistani officials reported that Habib had died in a US airstrike near the Afghan border, but Pakistani security officials have since retracted that claim, saying that no al-Qaeda leaders died.
Habib seems to have assumed overall command after al-Iraqi'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.Saif al-Adel
An Egyptian in his late 40s or early 50s, Saif al-Adel is the nom-de-guerreof 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 being a member of the group which assassinated former Egyptian President Anwar Sadat in 1981.
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.
He is believed to have been involved 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.
Following the invasion of Afghanistan in 2001, 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.
Abu Ghaith was arrested in Jordan and extradited to the US in March 2013.
Recent reports say Adel may have been released and made his way to northern Pakistan, along with Saad Bin Laden - but a report from 2011 suggested he had returned to Iran.
The US has offered up to $5m for information on his whereabouts.
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