SUMMARYThe urban air pollution is a very complicated problem. The exhaust emissions from internal-combustion engines account for a major portion of this problem. It is realized that the content and concentrations of the exhaust emissions depend on various parameters. These parameters include engine design parameters, operational parameters, exhaust gas aftertreatment, fuel types, fuel additives and lubricants. The present review paper discusses the effect of some parameters on the emission level and characteristics from internal-combustion engines. The paper begins with an introduction of general information on the nature of emissions of exhaust gases, including the toxicity and causes of emissions for both spark-ignition and diesel engines. The paper then shifts to an up-to-date information of the published research work on the subject matter.1998 John Wiley & Sons, Ltd.