The mechanisms described for soot formation have been reviewed. It is clear that no one simple mechanism can yet totally describe the complex processes in volved. It has been well established that ions have an important role in the nucleation stage, but controversy still surrounds the extent of their importance as precursors of soot. In the case of polymeric fuels, the gaseous fuels produced by thermal breakdown of the polymer are small organic molecules, which will then, generally, produce soot by mechanisms similar to those for fuels originally gaseous.