Prototype selective catalytic reduction (SCR) systems using urea have been demonstrated on diesel trucks in Europe in recent years. In view of upcoming stringent emissions control standards for US HD diesel engines, urea-SCR is being evaluated by US engine and truck manufacturers. The authors and their companies have worked jointly on a project to develop, test, and demonstrate urea-SCR on a US HD diesel engine and Class-8 truck.A prototype urea-SCR system was applied to a 12-liter HD diesel engine. The engine model selected is rated at 350 bhp and is common for highway trucks. The only engine modifications were changes to the injection timing control map in order to better suit the application of the urea-SCR system.This paper details two demonstration phases of the project as follows.The first phase includes recent emissions cell tests using a new compact SCR catalyst and an engine calibration optimized for lower NOX. In addition to NOX, CO, CO2, HC, and PM, NH3 was measured to evaluate slip through the catalyst. This latest data using the new catalyst will show the potential NOX reduction capability of this technology during emissions test cycles.The second phase includes application of an SCR system to a Class-8 highway truck. This work was completed using an earlier catalyst design and an engine calibration optimized for best fuel economy at higher engine-out NOX. Prototype NOX sensors were mounted in the exhaust stream before and after the catalyst for over-the-road measurements. NOX, urea consumption, and fuel consumption were recorded. These tests were completed in order to assess the effectiveness of this technology during actual truck operation.The results of this study showed substantial NOX reduction in engine cell and truck tests. The over-the-road measurements provide insight into real world NOX reduction and urea consumption. Overall, this study provided convincing evidence that urea-SCR may be a key technology to meet future emissions standards.