Intraepithelial neoplasia is of critical importance to the cancer chemoprevention field because it is a target condition for which drugs must be sought that will prevent its development or stop its progression. The term "dysplasia" refers to the morphological alterations that characterize intraepithelial neoplasia and according to many authors consists of seven basic morphological changes that occur in the majority of human epithelia, as well as in the epithelium of mouse skin papillomas induced by 7,12-dimethylbenz(a)anthracene and 12-O-tetradecanoylphorbol-13-acetate: increased nuclear size; altered nuclear shape; increased nuclear stain uptake; nuclear pleomorphism (increased variation in nuclear size, shape, and stain uptake); increased mitoses; abnormal mitoses; and disordered or absent maturation. Clonal evolution appears to begin early in the neoplastic process during intraepithelial neoplasia. Aneuploidy has been found during intraepithelial neoplasia in many human epithelia, and, in association with other forms of genetic instability, may provide the increase in genetically variant cells required for clonal evolution to occur. It is postulated that two major factors affecting the rate of progression of intraepithelial neoplasia are the cellular mutation rate, which is enhanced by environmental carcinogens, and the cellular proliferation rate, which is enhanced by agents that include sex hormones, inducers of chronic inflammation, and irritant chemicals which stimulate reactive hyperproliferation. A preferred chemoprevention strategy should consist of the development of drugs and drug combinations which will block mutagenic carcinogens or prevent epithelial hyperproliferation or its causes. Two examples of the induction of regression of intraepithelial neoplasia by chemopreventive drugs are the regression of oral leukoplakia produced by beta-carotene and the regression of colorectal polyps in patients with familial polyposis produced by sulindac. It is evident that there is a strong need for more research on the induction of regression of intraepithelial neoplasia with chemopreventive agents. There is also a critical need to identify and develop biomarkers that correlate with the appearance and regression of intraepithelial neoplasia.