The Islamist rebels among opposition fighters battling the regime of Syria's President Bashar al-Assad include independents, Muslim Brotherhood allies, Salafists and the jihadists of Al-Nusra Front, which has pledged alliance to Al-Qaeda leader Ayman al-Zawahiri.
Here are some of the key groups:
Syrian Islamic Liberation Front (SILF)
The SILF groups some 20 brigades and battalions, including some of Syria's most seasoned, such as Liwa al-Tawhid, the Farouq Brigades and Liwa al-Islam.
Though SILF members have an Islamist outlook, they have decided against advocating the establishment of an Islamic state while Assad remains in power.
Many SILF brigades started out as local, non-ideological battalions, but have grown more ideological as the conflict drags on.
Syrian Islamic Front (SIF)
The SIF, a smaller and tighter alliance set up in December 2012, is dominated by the Salafist Ahrar al-Sham movement.
Despite the hardline views of its members, the group is generally better regarded by Syrians on the ground than Al-Nusra Front.
Ahrar al-Sham calls openly for the establishment of an Islamic state after the fall of the regime, but the alliance has won respect across the board for its military abilities.
Its fighters were instrumental in the March 2013 insurgent takeover of Raqa, the first provincial capital to fall out of regime control.
Founded largely by foreign fighters, Al-Nusra subsequently began recruiting locally, aided by its reputation for discipline and military success.
In December 2012, the United States listed the group as a "terrorist" organisation because of its suspected ties to Al-Qaeda's Iraqi branch, the Islamic State of Iraq.
Those suspicions were confirmed when the ISI's chief announced Al-Nusra was part of its network.
Little is known about the group, but it has developed a reputation for military prowess and being well-equipped despite relatively small numbers.
The group has claimed responsibility for suicide bomb attacks and been criticised for using the indiscriminate tactic. Its leader's pledge of allegiance to Al-Qaeda was condemned by some rebel forces and activists.