Asthma is a chronic lung disease characterized by inflammation and reversible obstruction of the airways combined with episodic wheezing, coughing, or difficulty breathing. In some patients, chronic inflammation of the airway leads to permanent tissue damage and impairment of lung function. During acute asthma attacks, the bronchi can narrow substantially, primarily due to the contraction of smooth airway muscle. Reversal of this process typically is achieved with drugs that promote relaxation of airway muscle. More than 22 million Americans have asthma, and it is one of the most common chronic diseases of childhood, affecting an estimated 6 million children. Standard therapy depends on disease severity and consists of avoidance of allergens, and oral and inhalation bronchodilators and corticosteroids. Successful avoidance of asthma exacerbations depends upon patient compliance with therapy.