Carcinoma in situ is defined as the early stage of cancer and must therefore be initiated by an as yet unknown carcinogen(s). Progression of the lesion to invasive carcinoma is reported to occur in a high proportion of nontreated cases. Reserve cell proliferations are frequently associated with both dysplasia and carcinoma in situ, and it is suggested that these are the cells from which both lesions arise. Dysplasia may result from both carcinogenic and noncarcinogenic stimuli. Since dysplasia usually either regresses or remains stabilized over a prolonged period, it is suggested that it is more frequently associated with noncarcinogenic stimuli. Microinvasive carcinoma is limited to lesions with no more than 5 mm. of stromal invasion as measured from the surface. Confluence of growth and lymphatic-like space invasion should not interdict the diagnosis. Microinvasive carcinoma thus defined rarely gives rise to lymph node metastasis or eventuates in death. The diagnosis cannot be made from punch biopsy specimens. Only if pathologists adhere to a standard nomenclature can follow-up studies be used successfully to identify the natural behavior of each type of lesion in this spectrum.