For reasons not well understood, the prevalence of asthma, as well as its associated morbidity and mortality, has been increasing worldwide over the past 10 to 15 years. The pathophysiologic mechanism currently thought to underlie asthma involves both airway inflammation and bronchial hyperresponsiveness. Children who have asthma experience acute exacerbations of their disease, often precipitated by viral respiratory infections, by exposure to allergens or irritants, and by exercise. Wheezing, shortness of breath, chest tightness, and cough, all common symptoms that lead patients to seek relief, result at least in part from spasmodic contraction of bronchial smooth muscle. Drugs that relax bronchial smooth muscle produce the most immediate effect on airway caliber, even though edema and mucous secretions also contribute to airway obstruction.