This chapter covers the precancerous lesions for endometrioid and serous carcinoma of the endometrium, with some additional coverage of the limited amount of available information on possible precancer for endometrial clear cell carcinoma. For endometrioid carcinoma, the precancerous lesion is atypical hyperplasia/endometrioid intraepithelial neoplasia, clinicopathologic features of which are discussed in detail. There is also coverage of the spectrum of endometrial alterations associated with prolonged unopposed estrogen exposure. For serous carcinoma, the precancer is endometrial glandular dysplasia (EmGD). The current state of knowledge on the latter is addressed, with an emphasis on practical diagnostic applications. Serous endometrial intraepithelial carcinoma (SEIC) may display extrauterine extension and cause patient mortality and is accordingly not considered to be a precancerous lesion. For endometrial clear cell carcinoma, intraepithelial growth patterns, which may be seen in isolation, have long been recognized; whether these are precursor lesions is unclear. A previously proffered precursor lesion for clear cell carcinoma requires additional evidentiary support.