Currently, throughout the world combustion engine development is influenced by two primary concerns. First is the increasing concern for global warming, and second is the concern over particulate and oxides of nitrogen emissions, each of which affect the environment and human health because of the particles' toxicity and ground level ozone production, respectively.To address the global warming issue, in late 1997, various nations approved the Kyoto Protocol to reduce CO2 emissions because of its identified contribution to the greenhouse effect. The Diesel engine is the most efficient power plant for mobile and stationary purposes and, thus, Diesel engines are considered to be one alternative to gasoline engines to reduce fuel consumption and, thus, CO2 emissions.To address the emission concerns, the European Community and the U.S. Environmental Protection Agency (EPA) proposed emissions standards prescribing substantial reductions of NOX and PM emissions [1,2]1. As a result of these proposed standards, reductions in particulate and NOX emissions have become major challenges in Diesel engine development. Unfortunately, in the paradigm of Diesel engine development, ultra low emissions and very low fuel consumption are two opposing objectives. Moreover, NOX reduction from lean exhaust is fundamentally difficult because of the excess oxygen present. Particulate mass emissions from modern Diesel engines are already on very low levels; however, recently particulate numbers rather than particulate mass have attracted much attention and, thus, further reductions are necessary.This paper describes new approaches to ultra low emission Diesel vehicles that comply with the proposed emission standards. Advanced exhaust gas aftertreatment technology, including Diesel particulate filters and NOX catalysts, have previously been subjected to detailed research. In this paper, some new approaches are described, and their potential to large-scale vehicle application is assessed.