The impact of the Californian regulations on Europe, and their applicability to European conditions, is considered with particular reference to the diesel engine. In Europe photochemical smog is scarcely a problem; the toxic effect of carbon monoxide is the greater hazard. The emission by diesel engines of this and other toxic gases is low, but the exhaust smoke emission may be objectionable. Theories of smoke formation in combustion are considered, and methods of smoke measurement are described, including their shortcomings. The present position of European antismoke legislation is outlined, and characteristics of the diesel engine controlling smoke emission are examined. Finally, the performance of antismoke fuel additives, which may allow uprating of 10-15%, and the problems associated with their use, are described.