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Recent years have seen dramatic reductions in gasoline and diesel sulfur concentrations in the United States, Europe, Japan and other countries. Many developing countries are evaluating the appropriate sulfur levels to choose for the future. This paper examines the current state of knowledge concerning the impact of fuel sulfur on exhaust emissions, and the sensitivity of exhaust aftertreatment technology to fuel sulfur. Gasoline vehicles achieve very low emissions through use of three-way catalysts. These systems are relatively insensitive to sulfur, being able to operate on levels of up to 500 ppm. Further reduction in sulfur will produce additional, small emission reductions. Diesel emissions may be reduced significantly using engine modifications, oxidation catalysts or exhaust gas recirculation, which may require sulfur levels of 500 ppm.