Since the beginning of the conflict in 2011, mapping exactly what is happening inside Syria has been very difficult.
Few international journalists are present and conditions are difficult and dangerous. Reports of violence often comes via accounts posted on social networking sites, accompanied by grainy camera phone footage.
But based on detailed analysis of rebel and regime activity since October 2011, a map showing an assessment of the general situation on the ground has been produced by Washington-based think tank, the Institute for the Study of War (ISW).
Reports from human rights and opposition activists, as well as from the official Syrian Arab News Agency (Sana) and international journalists in Syria, were all taken into account when compiling the map.
In roughly a quarter of incidents, ISW analysts were able to cross-reference accounts of fighting and events from official government and rebel sources.
The ISW says that the "rebel-held zones" on its map are under de facto Free Syrian Army (FSA) control.
The FSA operates as a highly mobile guerrilla force and frontlines and zones of control constantly shift.
When challenged by government forces, their main tactic is not to defend territory but to withdraw, with the aim of avoiding losses and preserving manpower.
Explaining the methodology behind the map, ISW analyst Joseph Holliday told the BBC: "The regime can go into 'rebel-held' areas if they choose, but only at great cost.
"The regime has isolated outposts within those areas, from which they routinely shell surrounding villages. However, these outposts don't have the ability to project ground forces outside their walls, and in many cases are resupplied only by air or not at all.
"What's more, the regime is collapsing these positions one at a time, and the rebels are massing on and over-running others."
The FSA became more co-ordinated in its operations over the summer, mounting attacks on both the capital, Damascus, and second city of Aleppo. Both sides have continued to manoeuvre for control of critical supply lines.
As government forces regained control of Aleppo's commercial hub and intensified their air strikes, rebels increased pressure on outlying military facilities, mounting major attacks on some air bases, the ISW says. The rebels have succeeded in shooting down a number of aircraft, over-running airfields and, in late October, stopping government aircraft from flying into and out of Aleppo International Airport.
The largest areas under rebel control are currently to the north and east of Aleppo and down the centre of the country between Idlib and Hama. Rebel forces also took Bdama, to the west of Idlib, on the road which connects Aleppo with the coast.
In early November, rebels overran government checkpoints in Saraqeb and a nearby airbase, both critical links in government lines of supply between Idlib and Aleppo.
The ISW says government forces have strong positions in the eastern city of Deir al-Zour, but the the rest of the province is in rebel hands. Government forces withdrew from Kurdish-majority areas to the north in July, and they have since been under the control of Kurdish groups.
Analysts say the success of rebel tactics has forced President Bashar al-Assad to divert forces from other parts of the country, allowing the FSA to gain greater influence if not complete control of these weakened areas.