Since the earliest days of the AIDS epidemic, clinicians have been concerned about the prevalence of depression among their patients. Epidemiologists, psychiatrists, psychologists, sociologists, and a broad array of other specialists have studied this topic, trying to determine the prevalence of depressive disorders and depressive symptoms as well as antecedents, correlates, and consequences. This review addresses the methodologic difficulties in determining depression prevalence, major findings regarding rates of disorder and correlates among different segments of the HIV community, effects of depression on HIV illness progression, psychopharmacologic and psychotherapeutic treatment findings, and behavioral effects of depression, such as its impact on medication adherence, employment, and risk behavior. Finally, the article summarizes international studies of depression prevalence in developing countries and the challenges regarding cross-national diagnostic definitions and measures.