Viral infections, particularly respiratory illnesses caused by rhinovirus, are the most common cause of asthma exacerbations in children and contribute in large part to asthma morbidity in adults. Epidemiologic studies and increasingly sophisticated viral detection methodologies have helped to define the role of rhinovirus as a potential causative agent in asthma exacerbations. Rhinovirus-induced lung disease is multifaceted and can be characterized in terms of a variety of physiologic, immunologic, and viral processes. The precise direct and indirect mechanisms of viral contribution to exacerbations must still be elucidated. Understanding them will have an impact on the design of future treatment modalities.