There are wide variations in the prevalence of asthma among different countries, but the epidemiologic evidence suggests that most variations are due to environmental and not genetic factors. In recent years, major advances have been made toward quantifying the risk associated with certain environmental factors, particularly allergen exposure, diet, and indoor and outdoor air pollutants, and with genetic factors, i.e., gender and a parental history of allergic disease, that are involved in causing asthma. It is hoped that this knowledge will be helpful in the design of interventions to reduce the incidence of asthma in the next generation of children. In the meantime, it is encouraging that advances in measuring allergen levels are now leading to the documentation of the clinically important sources of allergens and the effectiveness of avoidance measures.