The increased prevalence of asthma over the past quarter century has become a major public health problem for the industrialized world. Asthma is a disease process which has a strong heritable component which is impacted by multiple environmental factors. Given the rapid increase in asthma prevalence, it is difficult to ascribe the change to a genetic alteration. Therefore, the focus for understanding the changing prevalence of asthma must be on environmental factors. This article reviews factors which may contribute, in whole or in part, to the development of the disease process. In questioning whether it is possible to prevent development of a disease (primary prevention), it is critical to understand these factors. The environment may even have an impact on the fetus during intrauterine life. There does appear to be a "window of opportunity" in early life where a variety of factors, including food and inhalant allergen exposure, exposure to pollutants, and infection with both viral and bacterial agents, may be important in initiating the development of asthma and allergy. Potential approaches to primary prevention of asthma and allergy must consider each of these important factors. Given that asthma is a multifactorial disease with both complex genetic and environmental components, it is unlikely that any single intervention will significantly decrease the prevalence of asthma.