Clinical and experimental investigations indicate that respiratory viral infections are important triggers for asthma attacks. Viral upper respiratory infections have been associated with 80% of asthma exacerbations in children and 50% of all asthma episodes in adults. Human Rhinovirus (HRV) has been implicated as the most common virus associated with asthma episodes. The observation that the great majority of wheezing lower respiratory tract illnesses in early life are associated with acute viral infections suggests that viruses may also alter the development of the lungs or of the immune system, acting as co-factors for the inception of asthma. Whilst there is no doubt that viruses are important asthma exacerbation factors, the role of viral infections in the development of asthma still remains controversial.