Precise information on cell turnover in the lung is surprisingly limited and it is derived for the most part from the study of small rodents. The determination of cell cycle time is dependent upon the identification of a pure population of cells. This requirement poses a particular problem in the analysis of the mixed epithelial components of the tracheobronchial tree and with the miscellany of cells that populate the pulmonary interstitium. Cell turnover in general is related to the need for replacement of aging or injured cells. This explains the rapid epithelial turnover in the major airways when compared with the much slower replacement of cells in the more stable milieu of the alveoli. Bronchiolar cytokinetics poses a special problem in rodents, which, unlike humans, have no respiratory bronchioles. This particular problem demands continuing study with special emphasis on the kinetics of the several cell types that populate the intermediate channels linking the airways with the air sacs.