Mapping territorial possession is made more difficult by the tactics employed by the various rebel groups.
They operate as highly mobile guerrilla forces, and when threatened by government troops will not defend territory but rather tactically withdraw with the aim of preventing losses.Conflict origins
The conflict in Syria began as a series of major pro-reform demonstrations in key cities in early 2011.
These protests - and the Assad government's reaction to them - became increasingly violent. The administration suffered a wave of military defections as protests escalated into a full-scale rebellion.
The Free Syria Army announced its formation in July 2011, and mounted attacks on both Damascus and the second city, Aleppo.
Rebel groups succeeded in capturing some military bases and acquiring heavy weaponry.
Government forces later regained control of Aleppo's commercial hub and intensified air strikes, while rebels increased pressure on outlying military facilities. They mounted major attacks on some air bases and shooting down a number of aircraft.Current control
Aleppo remains divided, and both sides have continued to manoeuvre for control of critical supply lines.
The largest areas under rebel control are currently to the north and east of Aleppo and around Idlib.
In June 2013 government forces, backed by pro-Assad Hezbollah fighters from Lebanon, recaptured the strategically important town of Qusair, close to the Lebanese border.
In nearby Homs a month later, the neighbourhood of Khalidiya was recaptured by the government. Only the Old City of Homs and a few other districts are still held by the opposition.
Then, in August, footage emerged of a suspected chemical weapons attack on several Damascus suburbs. Activists and foreign governments said hundreds had been killed. Both sides blamed each other for the incident - potentially the deadliest of the conflict so far.
The situation on the ground remains very fluid and any analysis will always be based on an assessment of reports of fighting, troop and weapons movements and other incidents.
In 2012, the BBC produced a series of maps based on analysis by the Institute for the Study of War (ISW), a US-based think tank.
The ISW based their work on reports from human rights and opposition activists, as well as from the official Syrian Arab News Agency (Sana) and international journalists.
The map above, produced by the Syria Needs Analysis Project (SNAP), is based on information from the US government compiled from media sources.