Density of exhaust smoke from a diesel engine is a sensitive indication of engine overload and fuel economy. The various lines of development of smokemeters are outlined. It is concluded that, with closely controlled maintenance, calibration, installation, and operating procedures, the two meters most commonly used in Europe are capable of giving adequate results under full-load steady-state testing conditions. A completely new approach to instrumentation for measurement of vehicle smoke emission under road or variable speed conditions is needed if satisfactory rapid test methods are to be evolved.In a British Standard for Diesel Engines for Road Vehicles engine rating is linked to an exhaust smoke density limit. It has been shown experimentally, that in steady-state testing, subjective acceptability of the smoke emission can be related to measured density on a sliding scale basis related to emission rate.A recommendation is made that, for world-wide correlation, standards relating to exhaust density should be expressed in carbon (or solids) concentration, while control regulations should take account of the effect of engine size and speed in defining acceptable smoke density levels.