Within the mouse lung, basal cells are restricted to the tracheobronchial epithelium, which is also populated with secretory, ciliated, and neuroendocrine cells to form a pseudostratified epithelium. Beyond the most proximal bronchi, the airway of the mouse is composed of a simple columnar epithelium that consists of secretory, ciliated, and neuroendocrine cells. In human lungs basal cells are present in diminishing numbers all the way to the level of terminal bronchiole. Respiratory bronchioles are lined by a poorly characterized cuboidal epithelium that leads to the alveolar compartment lined with ATI and ATII cells. Putative progenitor cells have been identified using in vivo lineage-tracing experiments (Clara and basal cells) or proposed based on their capacity to proliferate and differentiate in culture (Clara, basal, and ATII cells). The epithelium of the mouse trachea and proximal bronchi is maintained by airway basal cells (39, 78). Within the bronchiolar region, a population of Clara cell secretory protein–expressing (CCSP-expressing) cells localized the neuroepithelial body in the proximal bronchiole and a population of naphthalene-resistant CCSP-expressing cells from the distal bronchiole of the mouse serve to regenerate their respective regional epithelia following cell-specific injury (66, 67). BASCs, at the bronchioalveolar duct junction, have been suggested to serve as progenitors with the capacity to renew both the bronchiolar and alveolar epithelium (68). Finally, it has been long believed that ATII cells serve as the progenitors that maintain the alveolar epithelium (73).