Air quality improvement in Los Angeles, California is reviewed with an emphasis on aspects that may inform air quality policy formulation in developing cities. In the mid-twentieth century the air quality in Los Angeles was degraded to an extent comparable to the worst found in developing cities today; ozone exceeded 600 ppb and annual average particulate matter 〈 10 μm reached -150 μg.m -3. Today's air quality is much better due to very effective emission controls; e.g., modem automobilcs emit about 1% of the hydrocarbons and carbon monoxide emitted by vehicles of 50 years ago. An overview is given of the emission control efforts in Los Angeles and their impact on anabient concentrations of primary and secondary pollutants: the costs and health benefits of these controls arc briefly summarized, Today's developing cities have new challenges that are discussed: the effects of regional pollution transport are much greater in countries with very high population densities: often very large current populations must be supplied with goods and services even while economic development and air quality concerns are addressed; and many of currently developing cities arc located in or close to the tropics where photochemical processing of pollution is expected to be more rapid than at higher latitudes. The air quality issues of Beijing are briefly compared and contrasted with those of Los Angeles, and the opportunities for co-benefits for climate and air quality improvement are pointed out.