Driven in part by concerns regarding global warming, there is a clear trend toward increased sales of light duty diesel vehicles in many parts of the world. This trend can result in many positive environmental benefits including low fuel consumption, and therefore low levels of CO2, low levels of exhaust CO and HC (especially during cold start conditions), and very low levels of evaporative hydrocarbons. However, increased diesel sales have a downside, relatively high NOx and particulate emissions. These pollutants continue to receive high priority attention in most areas of the world. As a result, countries around the world are increasingly tightening diesel regulations with the result that technology for reducing emissions continues to advance. Engine and combustion improvements have substantially reduced NOx and particulate from modern engines. Questions are being raised, however, regarding whether the number of particles are actually being reduced or are only being shifted to a smaller size range. The status of regulation to reduce diesel NOX and particulate will be reviewed and summarized.