The disparity and heterogeneity of findings in the asthma literature are daunting, reflecting the complex nature of the illness. A basic requirement for the demonstration of a causal relation between a given exposure and a disease is knowledge of the temporal sequence of events. Only exposures occurring before the first symptoms of an illness can influence its inception. In a large proportion of patients, asthma starts in the first years of life. New-onset asthma also occurs during puberty and later in adulthood, but many cases of asthma in adults represent the reappearance of symptoms in persons who were transiently free of problems during adolescence and young adulthood. The environmental exposure causing asthma is likely to be important during fetal life and the early years and during the period before the onset of asthma in adults and adolescents. Any potential risk factor must eventually inter- act with an underlying, genetically determined pathway to result in the manifestation of disease. We do not fully understand the underlying mechanisms of asthma; they are likely to involve both inflammation and control of airway tone and re- activity. Asthma is probably not one illness, but a syndrome. The disease has different phenotypes with respect to its course and prognosis and its association with atopy. In infants with transient wheezing and in toddlers with nonatopic wheezing, self-limited airway obstruction develops after viral infection, whereas chronic, persistent asthma is more likely to develop in those with IgE-mediated wheezing. Asthma in adults includes phenotypes not seen in childhood, such as aspirin-induced and occupational asthma. However, these different phenotypes cannot readily be distinguished clinically. The disease is analogous to anemia, in which a number of different pathways related to congenital defects, dietary deficiencies, chronic infections, cancers, and autoimmune reactions all result in pallor and fatigue in affected patients. Another aspect of the causation of asthma is the context in which exposures occur and inter- act with a subject’s individual pathophysiological pathways. Variation in these pathways is determined by genes. Environmental exposures interact with these pathways, and these interactions can be detected through interactions between genes and the environment. However, these interactions do not occur in isolation. Pathways have more than one component, and the function of each component is determined by the gene or genes contributing to and regulating the processes. Not one but several genes involved in the construction and regulation of pathways are likely to contribute to the expression of a disease. If in fact more than one pathway leads to the development of asthma (as is the case with anemia), then not only many genes with small individual contributions, but also many environmental factors and gene– environment interactions will eventually be found to contribute to disease manifestation. Hence, different contexts in which a number of different exposures interact with various genetic backgrounds in a range of racial or ethnic groups will eventually result in changes in the incidence of asthma as a result of changes in pathways involving atopy, airway inflammation, airway hyperresponsiveness, or other still unknown factors (Fig. 4). The challenge in the years to come will be to integrate complex interactions between multiple exposures and numerous genetic variants to achieve an understanding of the causation of asthma.