The effectiveness of catalyst performance is a function of the inlet exhaust gas temperature, gas flow rate, concentration of NO{sub x} and oxygen, and reductant quantity and species. Given this interrelationship, it becomes immediately clear that an integrated development approach is necessary. Such an approach is taken in this project. As such, the system development path is directed by an engine-catalyst engineering team. Of the tools at the engine engineer`s disposal the real-time aspects of computer assisted subsystem modeling is valuable. It will continue to be the case as ever more subtle improvements are needed to meet competitive performance, durability, and emission challenges. A review of recent prototype engines has shown that considerable improvements to base diesel engine technology are being made. For example, HSDI NO{sub x} has been reduced by a factor of two within the past ten years. However, additional substantial NO{sub x}/PM reduction is still required for the future. A viable lean NO{sub x} catalyst would be an attractive solution to this end. The results of recent high and low temperature catalyst developments were presented. High temperature base metal catalysts have been formulated to produce very good conversion efficiency and good thermal stability, albeit at temperatures nearmore » the upper range of diesel engine operation. Low temperature noble metal catalysts have been developed to provide performance of promising 4-way control but need increased NO{sub x} reduction efficiency.« less