Lower respiratory tract infections in children, including group, bronchiolitis, and bronchitis are frequently associated with recurrent episodes of wheezing. Different respiratory viruses assume greater importance at different ages of children. Respiratory syncytial virus is the most prevalent viral respiratory infection in preschool children, while rhinovirus is of increasing importance in older children. Asymptomatic virus shedding and mild respiratory infections do not provoke asthma symptoms nor do bacteria, except in association with sinusitis. Furthermore, epidemiologic studies strongly suggest that viral lower respiratory tract illness in early childhood is associated with pulmonary abnormalities, including bronchial hyperreactivity and peripheral airway obstruction that may persist for many years, and is possibly a cause of chronic airway obstruction in adulthood. Several different mechanisms have been identified by which respiratory viruses provoke asthma. No one single mechanism, however, adequately explains virus-induced asthma. Nonetheless, a common thread to these various proposed mechanisms is the ability of respiratory viruses to cause airway inflammation, either directly, through cytopathic effects, or indirectly, by increasing the inflammatory processes of respiratory cells. The consequence of these effects causes increased airway responsiveness and asthma.