While diesel vehicles have many benefits, continued growth in the diesel population raises concerns regarding the health and environmental effects of the high NOx and particulate emissions which could result. The concern over diesel particulate has increased in recent years. Firstly, the evidence indicating adverse effects from all particulate in urban air has grown stronger. Secondly, regarding diesel's cancer risk, evidence has grown that there is no threshold, i.e., there is no level below which one could consider diesel particulate to be safe. 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. Oxidation catalysts have also become widespread especially on passenger cars in Europe and trucks in the US. Particulate trap oxidizers continue to undergo development and refinement to reduce cost and complexity and are expected to become more widely utilized in the future, not only for new vehicles but also as retrofits for existing buses and trucks. Whatever future improvements emerge for reducing “engine out” emissions, aftertreatment technology which can reduce 50 to 90% of the remainder will be necessary since health concerns require diesel emissions to be reduced to the greatest degree feasible.