It is well established that ambient particles in the size range of 2.5 microns or less case a wide variety of adverse health effects. According to a recent study from the World Health Organization, in 2010 these effects resulted in approximately 3.2 million premature deaths with vehicles being one of the significant contributors. Diesel vehicle particulate emissions which are virtually all smaller than 2.5 microns raise additional special concerns due to their carcinogenicity and high ratio of black carbon (BC) to organic carbon; black carbon has recently been found to be the second most important contributor to climate change after carbon dioxide. Other pollutants emitted by diesels and other vehicles such as the oxides of nitrogen and volatile organic compounds also contribute to ambient particulate matter smaller than 2.5 microns in size (PM_2.5) after undergoing secondary transformations in the atmosphere. Technologies have been developed that can dramatically reduce vehicle emissions when clean, low sulfur fuels are available and these technologies are being phased in throughout the industrialized world resulting in a global decrease in particulate matter (PM) and BC emissions from vehicles. However the vehicle population is growing rapidly in the developing world, leading to increases in emissions in many countries. Unless these rapidly industrializing countries move to state of the art vehicles and clean fuels, global PM, BC and NO_x emissions from road vehicles will start to turn up over the next 10 to 15 years.