This chapter reviews the evidence that asthma is increasing and that changes in exposure to environmental risk factors may explain the increase. Although asthma is difficult to define for epidemiological studies, the prevalence of asthma as measured by the questionnaire definitions 'asthma ever diagnosed' and 'wheeze ever' is large and increasing. In all countries where serial studies using the same methods have been undertaken over the last 20 years, an increase in wheezing illness in children and adolescents has been recorded but there are insufficient data to determine whether the disease is increasing in adults. Despite the recorded increases, there remains a large difference in the prevalence of asthma between populations, with high rates of wheezing illness in Australasia and low rates in villages in poor countries. The male to female ratio for the occurrence of asthma remains at about 1.5 in children, 1.0 in late adolescence and less than 1.0 in adults, when more females than males have symptoms. The risk factors for childhood asthma are atopy (positive skin tests), parental asthma, allergen load, respiratory infections, some aspects of diet and an 'affluence' factor. There is some evidence for an increase in the prevalence of atopy in children but this may be due to earlier acquisition of atopy. Changes in the other risk factors have not been documented. The evidence for changes in indoor allergen loads, in diet, in the severity and nature of respiratory infections, and in 'affluence' is indirect and comes from a number of small studies rather than from serial epidemiological studies. It seems unlikely that a single, environmental risk factor has changed dramatically worldwide. Rather, a number of lifestyle changes may have combined to cause the disease to be expressed in children who, in previous times, were immunologically protected from developing asthma, perhaps by their T helper cell phenotype, or were not exposed to high allergen levels.