Lean combustion in spark-ignition engines has long been recognized as a means of both improving engine efficiency and lowering carbon monoxide and oxides of nitrogen exhaust emissions. In this paper, the fundamentals of lean combustion are reviewed in terms of lean flammability and ignition, and lean misfire limit, from a compilation of research results. 125 references are included in the review. The effect of lean mixture on engine efficiency, performance and exhaust emissions is discussed. The effects of engine variables on lean combustion are presented, including mixture preparation, ignition, combustion chamber shape, compression rates and fuel additives.Techniques for lean burn engine control and emissions clean-up are also described.