Airway inflammation, airflow obstruction, and bronchial hyperresponsiveness are characteristic phenotypic features of asthma. Clinically, airflow obstruction in asthma often is not fully reversible, and many asthmatic subjects experience an accelerated and progressive loss of lung function over time. Histopathologic studies of the asthmatic airway have demonstrated stereotypic changes that might explain the loss of lung function that many patients with asthma experience. The notion of airway remodeling in asthma postulates that the alteration of the structure and function of key airway constituents, including airway smooth muscle, epithelium, blood vessels, and mucus glands, might explain, at least in part, the progressive loss of lung function that is observed clinically. Inflammation driven by CD4(+) lymphocytes and mediated by effector cells, particularly the eosinophil, appears to modulate the function of mesenchymal cells, including fibroblasts and myofibroblasts, changing the composition of the airway wall matrix. Changes in the airway epithelium might alter the function of the underlying smooth muscle and the composition of the matrix and could drive inflammation. Alterations in the structure and function of airway smooth muscle change the mechanical properties of the airway wall and might also affect the function of other airway constituents. A variety of experimental models have identified candidate mechanisms and mediators for these observed changes, which are thus potential therapeutic targets. However, clinical studies to date have been disappointing, and it remains to be seen whether targeted therapies will prevent the progressive loss of lung function seen in asthma.