Viral respiratory tract infections have been epidemiologically associated with asthma in at least 3 ways. First, during infancy, certain viruses have been implicated in the inception of the asthmatic phenotype. Second, in patients with established asthma, particularly children, viral upper respiratory tract infections play a significant role in producing acute exacerbations of airway obstruction, often resulting in outpatient visits or hospitalizations. This increased propensity of viral infections to produce lower airway symptoms in asthmatic persons may be related, at least in part, to interactions among allergic sensitization, allergen exposure, and viral infections-all acting as cofactors in the induction of acute episodes of airflow obstruction. Third, and perhaps counterintuitively, certain infections may actually prevent the development of allergic respiratory tract diseases, including asthma. This review discusses these associations as they pertain to both the pathogenesis and treatment of childhood asthma.