Endometrial adenocarcinoma is the most common malignant tumor of the female genital tract in the United States. This neoplasm represents a biologically and morphologically diverse group of tumors, with differing pathogenesis. These tumors have two basic clinicopathologic forms, type I and type II. These two forms of endometrial carcinoma display different clinicopathologic, immunohistochemical, and molecular biologic features suggesting two pathways of carcinogenesis.