In children and adults with asthma, infections with common cold viruses such as respiratory syncytial virus and rhinovirus frequently cause exacerbations of asthma. Recent studies suggest that these and other respiratory viruses infect the lower airways, and that the infected epithelial cells help to initiate immune responses to the virus. These immune responses have antiviral effects, but also enhance airway inflammation and may add to cold symptoms and airway obstruction. Moreover, there are data to suggest that there may be specific interactions between allergic and virus-induced inflammatory responses that contribute to lower airway dysfunction in asthma. Finally, impaired T-helper type 1-like lymphocyte responses during viral infections may be a risk factor for more severe viral illnesses in patients with allergies and asthma. It is hoped that these advances in our understanding of the relationship between viral infections and exacerbations of asthma will translate into new therapeutic targets for virus-induced exacerbations of asthma.