Childhood asthma is a condition characterized by airflow obstruction that varies in time spontaneously, in response to various environmental stimuli and in response to treatment. Asthma is more common in childhood than in adulthood and is more correctly thought of as a syndrome than as a discrete condition. Asthma can be thought of as a failure of development where the normal development of the respiratory and immune systems is altered by the impacts of environmental exposures acting on underlying genetic predispositions. These organ systems are immature at birth and have prolonged periods of postnatal growth and development, making them particularly vulnerable to adverse environmental exposures. The consequence of a given exposure is determined by the developmental stage during which it occurs. Exposures occurring during organogenesis may result in structural abnormalities, whereas the same exposure occurring later may or may not have an effect on organ function. The ‘windows of susceptibility’ differ for different organs and exposures. Exposure to air toxins can limit lung growth and increase the risk of developing asthma. A synergistic interaction between allergic sensitization at an early age and lower respiratory viral infections in the first year of life markedly increase the risk of asthma in later childhood.