1ABSTRACTIn recent years Diesel DeNOx catalysts using additional hydrocarbons as reducing agents have been the focus of exhaust aftertreatment. The NOx reduction potential was often limited to 20 - 30 % in the European MVEG-A or the US FTP cycle by just adding a DeNOx catalyst on a vehicle. This result is explained by the fact that the catalyst was treated as a separate item and that the emission reduction strategy was not developed in a system approach. This paper summarizes results regarding the potential of state of the art Diesel DeNOx catalysts fitted to passenger cars and trucks when the exhaust gas system is optimized as a whole.The easiest way for a system approach is the combination of DeNOx catalysts with different working temperatures for NOx reduction. This has been demonstrated by the usage of several base metal catalysts for heavy duty applications. For passenger cars Platinum containing catalysts are strongly favored. In order to achieve high DeNOx activity a two catalyst concept has to be used. Hydrocarbons as reducing agents have to be present at both catalysts. This can be attained by multiple hydrocarbon injection or by applying a bypass solution. The catalyst type, the Platinum content and the catalyst location along the exhaust line has to be selected so that the DeNOx temperature window of the catalyst and the average gas temperature at the catalyst location fit well together. Temperature control of the exhaust gas before catalyst is one of the key parameters in this type of system development. The beneficial aspects of implementation of an exhaust gas cooler on the DeNOx performance will be shown. The type of hydrocarbon is important for the DeNOx performance as well. This can be controlled by the post injection timing using Common Rail. The usage of special diesel fuels is another alternative to use the suited hydrocarbons.