This review touches on what is being learned about wheezing in infants, how it differs from wheezing in later childhood, and what has been relearned about its natural history. Wheeze is a common symptom with a number of different causal mechanisms and has a better prognosis in infants than in older children. The current ability to measure the lung function of infants has clearly shown the harmful effects of exposure to tobacco smoke. These effects begin in utero and continue during infancy and childhood. Improved viral isolation techniques have confirmed the important role of infections with respiratory syncytial virus, rhinovirus, and adenovirus in wheezing episodes. In later childhood, allergen sensitization and allergen exposure become of increasing importance.