Endometrial hyperplasia is a heterogeneous set of pathologic lesions that range from mild, reversible glandular proliferations to direct cancer precursors. These lesions comprise a continuum of morphologic appearances, with the earliest proliferation represented by crowded glands with simple tubular architecture lined by cells resembling proliferative endometrium, whereas advanced proliferations in this continuum are characterized by crowded glands with complex architecture, often containing cells with nuclear atypia resembling low-grade endometrioid adenocarcinoma. The former "early" proliferations may be isolated to an endometrial polyp, but advanced proliferations are generally more diffusely present throughout the endometrium. There are at least three major classification systems for endometrial carcinoma precursor lesions, each of which trend toward overlap at the complex end of the spectrum. Although some classifications are based on a series of molecular genetic alterations (which may or may not translate into biologically or clinically relevant risk lesions), each classification scheme ultimately uses a series of histologic features, usually a combination of architecture and cytology, to establish a diagnosis of hyperplasia. Because different pathologists may apply different histologic criteria for endometrial hyperplasia depending on the classification system used, this article will provide an overview of the classifications used in current daily practice, present the histologic criteria and relative merits of each classification system, and discuss common and not so common causes of misclassification.