AbstractFor epithelial cancers which account for the majority of human malignancies chemoprevention by definition, consists of intervention measures taken during the develop ment of the neoplastic process up to the stage of in situ carcinoma. When considering chemopreventive strategies, at least two phases of the epithelial neoplastic process must conceptually be distinguished. Firstly, the initation and early reversible promotion phase, secondly, the later more autonomous phase of tumor development. We suggest that these distinctions are of crucial importance as the effective dose of chemopreventive agents may differ, depending on the stage of tumor formation. Failure to take these considerations into account could result in negative or inconclusive chemoprevention trials.