The paper describes research and development work concerning the investigation and optimization of lean NOx catalysts for gasoline and diesel engine applications. The investigations carried out in the laboratory, the engine bench and the chassis dynamometer demonstrate the influence of a number of important catalyst design parameters on the catalyst activity for nitrogen oxide reduction as well as for the conversion of carbon monoxide, gaseous hydrocarbons and in the case of diesel engine exhaust for particulates. In the first part of the paper significant improvements in catalyst activity and durability compared to the first catalyst generation are outlined. A major part of the paper deals with the influence of operation parameters such as space velocity and linear velocity of the reactants in the catalyst. Other aspects discussed are the influence of exhaust gas hydrocarbon component, active site distribution and poisoning elements affecting catalyst activity and durability. Finally, promising results on the performance of a improved catalyst system design in chassis dynamometer and engine bench tests are given.