Stratified squamous epithelia, such as those covering the skin, esophagus, and cervix, are normally in a dynamic steady state: production of new cells (proliferation) is matched by loss of terminally differentiated cells into the environment (desquamation). The parameters that describe population dynamics in stratified epithelia--number of dividing cells, number of cell layers, transit time, and rate of desquamation--can be closely monitored in cultures of stratified epithelial cells. Analysis of these data show that cultures of stratified epithelial cells can be maintained in a dynamic steady state for at least 1 month and thus have a dynamic behavior similar to stratified epithelia in vivo. Although this in vitro behavior may be intuitively reasonable based on the in vivo behavior of these cells, it is remarkable in that it is contrary to the general experience with other normal cell types in culture. The usefulness of measuring population dynamics in cultures is demonstrated by an analysis of the actions of retinoids on human keratinocytes. In addition, we show that because of favorable geometry and ease of manipulation, these cultures are well-suited to the analysis of heterogeneity in the proliferating population of cells.