In Part A, the techniques most commonly used to study epithelial cell kinetics and their suitability for use in man are reviewed. In Part B, the application of such methods to comparative studies of normal and abnormal oral epithelial of both man and experimental animals is examined. Claims regarding the possible prognostic value of LI's or MI's of biopsies of potentially or overtly malignant human oral lesions are also considered. Normal and abnormal kinetic data for other lining epithelia, e.g., skin, uterine cervix and gastrointestinal mucosa are assessed where they provide a better illustration of some of the problems arising in such comparative investigations. Finally, the present and likely future areas in which cell kinetic studies may provide a guide to expected tumour prognosis and optimum treatment are summarized.