In this monograph, non-neoplastic endometrial lesions and their differential diagnosis are reviewed. The normal menstrual cycle, secretory and non-secretory lesions are reviewed pictorially. The minimal clinical information required for interpretation in most cases includes the patient's age, the date and characteristics of the last menstrual period, a statement about past or current administration of steroids, and an account of the current chief complaint and physical finding. A knowledge of these subjects can help the pathologist to understand the problems of the patient and the gynecologist.