The health effects of air pollution have been extensively studied and are becoming increasingly well understood. In epidemiology, most studies have focused on the short- and long-term effects of background urban pollution on the general population; some have focused on the individual exposures of specific populations due to their occupational exposure, health status, or age (drivers, patients with asthma or diabetes, children, or the elderly). The contribution of these studies to our understanding of the health effects of air pollution has been and continues to be significant. These studies have also made it possible to measure the impacts of pollution in terms of public health (number of deaths, impact on life expectancy, number of hospitalizations, and medical visits) and economics (cost-benefit analyses of measures aimed at reducing pollution). They have been instrumental in supporting national regulatory decisions (Law No. 96-1236 of December 30, 1996, on Air and the Rational Use of Energy in France—LAURE) and European directives (European Air Quality Directives) aimed at reducing pollution levels. And they have contributed to the development of local anti-pollution policies (urban planning and transportation plans). However, it must be noted that despite the knowledge generated, the implementation of measures to reduce pollution levels remains limited. To provide more effective support for decision-making in this area, epidemiology currently proposes three areas of focus: an approach based on specific sources of pollution, the study of the chemical composition of pollution indicators, and so-called "intervention" studies. (Introduction)