:Over the past century, huge amounts of air pollutants and gases have been released into the atmosphere that now pose risks to human health, natural ecosystems, and the earth's climate. Motor vehicles are a major contributor to both the build-up of greenhouse gases -- potentially the most serious of these problems -- and the creation of urban smog and acid rain. The purpose of this paper is to summarize the nature of these problems and the role played by motor vehicles in causing them. The significance of pollutants other than CO2 which are emitted in large quantities from vehicles will be shown to be important for global warming. Further, the increasing significance of vehicles produced or registered in rapidly expanding countries will be assessed. Finally, the growing importance of vehicle modes other than passenger cars will be summarized.