Profound change has been observed in automotive emissions since the late 1960's. This change is marked both in the amount and in the chemical make-up of the controlled emissions. In general, the trend toward decreased levels of the several categories of emissions is well known, but changes in the chemical character of the hydrocarbon component are less well recognized. The nature of these changes and implications for air pollution effects are discussed.As the absolute levels of emissions are reduced, small incremental changes become relatively more important. Such incremental changes are brought about by change in ambient temperature, and therefore the sensitivity of emission control systems to changes in ambient temperature takes on added significance. Data are presented to show that emissions control may deteriorate seriously at some temperatures at which vehicles normally operate.Will emissions considerations force change in fuel usage, and if so, how are emissions affected? The question is examined both from the viewpoint of a switch from gasoline to other conventional fuels and from the viewpoint that gasoline composition may change. Moreover, with supplies of motor fuel from petroleum sources in long-term jeopardy, there is growing interest in the character of emissions to be expected in using alternative fuels. The more viable options for alternative fuels are identified, and emissions characteristics of these fuels are described.