Air pollutant emissions from mobile sources have been regulated for almost half a century. During this time, the focus has largely been on tightening emissions standards for on-road vehicles and engines, particularly passenger cars and small trucks (light-duty vehicles). Light-duty-vehicle emissions control grew out of research that implicated the increasing use of vehicles in the deterioration of air quality conditions in the 1950s in Southern California. Control of motor vehicle emissions began in the early 1960s with the introduction of positive crankcase ventilation, a simple approach consisting of a hose and valve that reduced the venting of uncombusted gases to the atmosphere. From that simple beginning, light-duty-vehicle emissions control evolved to todays complex regulation of fuel properties, exhaust emissions, and evaporative emissions, which require the use of sophisticated engine and emissions-control technologies. These strategies enabled per-mile-exhaust emissions of new, properly operating light-duty vehicles to decrease by 95- 99% in 2004 compared with emissions of 1967 model-year vehicles. The focus of mobile-source emissions control expanded to include on-road heavy-duty engines and later nonroad engines. The broadening in regulatory attention arose from the increasing fraction of mobile- source emissions that come from sources other than light-duty vehicles and the relative lack of emissions controls on these sources. On-road heavy-duty-vehicle engines were first regulated for air pollutants in the 1970s, and engines used in off-road applications were first regulated in the mid-1990s. Over the next decade, regulations already approved for new sources will substantially reduce emissions from on-road diesel vehicles, nonroad diesel engines, and gasoline-powered nonroad engines.