In the U.S. we have reached the point where further reductions in per-mile emissions from individual mobile sources of criteria pollutants will be both tiny and expensive. In addition, as population grows, total mobile source emissions in developed countries are likely to increase as our ability to engineer reductions on a vehicle-by-vehicle basis reaches its technological limit and is overwhelmed by the rising numbers of miles driven. Mobile source emissions world-wide will climb as greater wealth in the developing world fuels the demand for mobility. This article examines the demand for transportation and the regulation of transportation fuels and then assesses the possible steps for future regulation. As to pollutants where the issue is total loading in the atmosphere (e.g., CO2), the author argues that it will be cheaper and more effective to buy offsets in the developing world than to attempt to reduce emissions only within the developed world. The author further argues for incentives to induce changes in individual driver behavior in place of command and control measures and for changing anti-trust regulation to allow for tighter integration of fuels and engines to reduce mobile source emissions. Even with these measures, however, the author argues that stationary source regulation is going to have to pick up a larger portion of future gains in air quality.