Cohort studies, which are longitudinal studies that follow a group of people with reference to the development of disease, have been a cornerstone of research on childhood asthma. These studies are uniquely suited to address questions concerning the incidence of illness, the natural history of disease, and the sequence of events linking exposures with outcomes. Three findings from on-going cohort studies are particularly relevant for the design of future intervention studies. First, most childhood asthma begins in infancy, with 80% of children who develop asthma having their first episode of wheeze before the age of 3 yrs. Second, events in early life, possibly including allergen exposure, infant feeding practices and viral infections, may be critical to the development of asthma in childhood. Finally, wheezing presents as separate phenotypes at different ages, with each phenotype having distinct characteristics, risk factors and prognoses. Additional cohort studies are required to determine to what extent events occurring in infancy, both viral and allergic, trigger expression of asthma, what are the mechanisms whereby they foster development of the disease, and whether their effect can be prevented.