Natural gas has considerable potential as a “clean” fuel for motor vehicles. This paper reviews the present state of the art in natural gas vehicles, focusing on engine technology and emissions. Natural gas engines generally show very low emissions of reactive hydrocarbons, carbon monoxide, and particulate matter, but NOx emissions can be fairly high. Approaches to NOx control include stoichiometric operation using a three-way catalyst and air-fuel ratio feedback, and “lean-burn/fast-burn” engines with NOx controlled through reduced flame temperature. Engines of both types have demonstrated excellent emissions performance - exceeding U.S. 1994 emissions requirements for heavy-duty engines. Optimized natural gas engines are significantly more efficient than present gasoline engines, with lean-burn natural gas engines approaching diesel efficiency.