Epidemiological studies performed on children and young adults indicate that the prevalence of asthma has increased over the past decades in a number of countries. Since genetic determinants alone are unlikely to explain these findings, environmental factors are expected to have an important impact on the prevalence of this disease. Environmental pollution, among many other factors, may play a role in the initiation and manifestation of asthma. However, there is little epidemiological evidence for a causal association between the classical major outdoor air pollutants, such as SO2 or particulate matter, and asthma. Evidence for an association between air pollution from motor vehicles and asthma is still controversial. Thus the indoor environment and other environmental determinants--including lifestyle factors--are likely to be more important as causes of asthma. Epidemiological studies are able to evaluate the effects of environmental factors on the health of human populations in their usual environments. In addition, they can measure associations between long-term environmental exposures and health outcomes. By using refined techniques for exposure and disease assessment, as well as time and cost-effective study designs, environmental epidemiology will increase the knowledge about the role of pollution as a possible cause of asthma.