I. Introduction The epithelial lining of airways is a complex structure that both protects the airway and regulates its function. The epithelium serves as a barrier to the external environment and makes substantial contributions to bronchial airway function. It regulates fluid and ion transport across the airway and into the airway lumen, may modulate airway smooth muscle tone, stimulates an influx of inflammatory cells during injury and inflammation, and secretes numerous mediators, both inflammatory and trophic. The epithelium may be a specific target of inflammation in asthma: damage to the epithelium, combined with perturbation of its function, may contribute substantially to airway inflammation, edema formation, mucous plugging, and bronchoconstriction. It is not clear whether
dam~~~~~um~~~~~M~m~~~~~~ process that leads to asthma. That is, is the epithelial layer simply a target for other cells and for inflammation, or does it participate in the genesis of airway inflammation? This chapter focuses primarily on the epithelium as a target organ in asthma and examines the morphological and functional consequences of such targeting.