Precancerous stages are precursors of a potential malignant growth which lead to cancer with a certain regularity. They can occur in all organs and all tissues. Consequently, they play an important part in the prevention and early detection of cancer in all medical specialties. For the pathogenesis of precancerous stages as morphologically well defined preneoplastic tissue changes ("precancerous lesion") or as disorders with risk of cancer lacking morphologic preneoplastic changes ("precancerous condition"), an individual disposition (genetic, racial and immunological factors) and an exposure against chemicophysical noxae are responsible. In view of the therapeutical procedure the cancer incidence rate, the latency period up to the appearance of malignant growth as well as the localization and the extension of the disorder have to be weighed against morbidity and mortality due to a presumptive treatment. Easily accessible precancerous stages, like some skin diseases and polyps of the gastrointestinal tract have to be cleaned up. If conservative or surgical treatment is not possible or not convenient, or if exposure against carcinogenic noxae cannot be avoided, the risk patients must undergo careful observation for the prevention of cancer.