There exists a spectrum of potential causes of abnormal uterine bleeding (AUB) in nonpregnant reproductive-aged women. In some instances the abnormal bleeding may emanate from definable pathologic entities in the endometrial cavity such as polyps or submucous leiomyomas (fibroids), while in others, it may occur secondary to coagulopathies or disorders of local or systemic hemostasis. If the patient is subjected to a rigorous evaluation more than one potential contributor may be identified, a circumstance that challenges the clinician since entities such as polyps, leiomyomas, and adenomyosis may frequently be asymptomatic, even in women with AUB. The bench or clinical investigator may also be influenced by the existence of the multiplicity of potential causes that could confound the results of research. This paper describes FIGO's (Fédération Internationale de Gynécologie et d'Obstétrique) new PALM-COEIN classification of causes of AUB in the reproductive years (Polyp, Adenomyosis, Leiomyoma, Malignancy and hyperplasia-Coagulopathy, Ovulatory disorder, Endometrial, Iatrogenic, Not yet classified). This system is designed to assist clinicians in the evaluation of patients, investigators in the design and interpretation of research and faculty in the education of medical students and residents/trainees. Additionally, given the longstanding existence of a confusing array of conflicting definitions, the manuscript describes the FIGO standardized terminology for both normal menstruation and AUB symptoms.