The prevalence of childhood asthma appears to be increasing worldwide. A critical element in the development of childhood asthma is maturation of the child's immune system. Most asthmatic children have a history of recurrent lower respiratory tract illnesses associated with airway obstruction during the first year of life. Most infants and young children who will go on to have persistent wheezing and asthma show high IgE production and eosinophilic immune responses at the time of their first viral lower respiratory tract illness. Understanding the genetic and environmental factors that regulate the maturation of the immune response during early life will greatly enhance the development of strategies for the primary and secondary prevention of asthma.