Current, state of the art natural gas engines provide the lowest emission commercial technology for use in medium heavy duty vehicles. NOx emission levels are 25 to 50% lower than state of the art diesel engines and PM levels are 90% lower than non-filter equipped diesels. Yet, in common with diesel engines, natural gas engines are challenged to become even cleaner and more efficient to meet environmental and end-user demands. Cummins Westport is developing two streams of technologies to achieve these goals for medium-heavy and heavy-heavy duty applications. For medium-heavy duty applications, lowest possible emissions are sought on SI engines without significant increase in complexity and with improvements in efficiency and BMEP. The selected path builds on the capabilities of the CWI Plus technology and recent diesel engine advances in NOx controls, providing potential to reduce emissions to 2010 values in an accelerated manner and without the use of Selective Catalytic Reduction or NOx Storage and Reduction technology. For heavy-heavy duty applications where high torque and fuel economy are of prime concern, the Westport-Cycle{trademark} technology is in field trial. This technology incorporates High Pressure Direct Injection (HPDI{trademark}) of natural gas with a diesel pilot ignition source. Both fuels are delivered through a single, dual common rail injector. The operating cycle is entirely unthrottled and maintains the high compression ratio of a diesel engine. As a result of burning 95% natural gas rather than diesel fuel, NOx emissions are halved and PM is reduced by around 70%. High levels of EGR can be applied while maintaining high combustion efficiency, resulting in extremely low NOx potential. Some recent studies have indicated that DPF-equipped diesels emit less nanoparticles than some natural gas vehicles [1]. It must be understood that the ultrafine particles emitted from SI natural gas engines are generally accepted to consist predominantly of VOCs [2], and that lubricating oil is a major contributor. Fitting an oxidation catalyst to the natural gas engine leads to a reduction in nanoparticles emissions in comparison to engines without aftertreatment [2,3,4]. In 2001, the Cummins Westport Plus technology was introduced with the C Gas Plus engine, a popular choice for transit bus applications. This incorporates drive by wire, fully integrated, closed loop electronic controls and a standard oxidation catalyst for all applications. The B Gas Plus and the B Propane Plus engines, with application in shuttle and school buses were launched in 2002 and 2003. The gas-specific oxidation catalyst operates in concert with an optimized ring-pack and liner combination to reduce total particulate mass below 0.01g/bhphr, combat ultrafine particles and control VOC emissions.