With regard to increasingly stringent emission legislation natural gas is gaining interest as an alternative fuel. Concerning mobile application natural gas is often considered to produce potentially lower exhaust emissions compared to diesel and gasoline fuel. Nevertheless, also the exhaust gas quality of diesel and gasoline fuelled vehicles will be improved by applying advanced technical solutions.The paper reveals the state-of-the-art in exhaust emission behaviour of diesel, gasoline and natural gas fuelled vehicles. Passenger cars and light-duty trucks will be considered as well as HD-trucks. Emissions include NOx, THC, NMHC, CO, Aldehydes and PAH. In additon CH4 and CO2 emissions are discussed with respect to increasing concern about the greenhouse effect. Special emphasis is laid on the role of mixture formation systems, catalysts and gas composition for natural gas engines. Regarding future emission legislation the potential of diesel, gasoline and natural gas fuels will be evaluated. New technical solutions mill be taken into account and their impact on fuel consumption will be estimated.