Riyad Al Assaad, Free Syrian Army
A former colonel in the Syrian air force who defected in July 2011, Mr Al Assaad is the self-declared leader of the Free Syrian Army (FSA), a loosely affiliated group of defected soldiers and other fighters who make up the main armed opposition to the Al Assad regime.
His leadership has come under some dispute as another, higher-ranking defected officer is now claiming to lead the rebels' military council.
Burhan Ghalioun, Syrian National Council
A Paris-based sociologist and a pro-democracy advocate since the 1970s, Mr Ghalioun was named in September 2011 to head the Syrian National Council (SNC), an umbrella for Syrian opposition groups that was formed in Istanbul the previous month.
The SNC has come under mounting pressure from within Syria for not overtly backing armed resistance against Mr Assad, which is being led by the FSA.
Mr Ghalioun's critics say he has failed to provide authoritative leadership for the disparate factions of the SNC.
Haitham Al Maleh, Syrian Patriotic Group
The former judge has opposed dynastic rule by Mr Al Assad and his father, the late Hafez Al Assad, since the 1970s.
The organisation he heads, the Syrian Patriotic Group (SPG), is made up of at least 20 former secular and Islamist members of the SNC.
The SPG was formed at the "Friends of Syria" gathering in Tunis last weekend to protest against what it called the SNC's failure to achieve "satisfactory results or activate its executive offices or adopt the demands of the rebels inside Syria".
The SPG said it backed "the national effort to bring down the regime with all available resistance means".
Hassan Abdul Azeem, National Co-ordination Committee
A long-standing critic of the regime, Mr Hassan Abdul Azeem heads the National Co-ordination Committee, a group of dissidents who urge dialogue and reject violence and foreign intervention.
Critics say the group out of step with the mood of the street.