Endometrial hyperplasia (EH), with or without atypia, is a common gynecologic diagnosis and a known precursor of endometrial carcinoma, the most common gynecologic malignancy. During the reproductive years, the risk of EH is increased by conditions associated with intermittent or absent ovulation, in particular, polycystic ovary syndrome. After menopause when ovulation has ceased, EH is more common in women with conditions that increase levels of circulating estrogen such as obesity or estrogen replacement therapy. Women with EH are at increased risk for both concurrent and subsequent endometrial cancer. The risk of coexisting cancer in women with a diagnosis of EH at endometrial sampling is due to limitations in both endometrial sampling and the diagnostic reproducibility among pathologists. These diagnostic uncertainties add to the complexity of managing EH. This review offers a rational approach to prevention, diagnosis, and treatment of EH, including hormone therapy and conservative surgical methods. © 2012 AAGL.