The essential meaning of apathy is lack of motivation. Criteria for diagnosing the syndrome of apathy and for distinguishing symptomatic and syndromal apathetic states are based on this definition, whereas assessment of apathy as a continuous variable is operationalized in the Apathy Evaluation Scale, which provides reliable and valid measures of diminished motivation in diverse clinical populations. The clinical utility of this formulation is shown by describing the causes, differential diagnosis, and treatment of apathy. The neural mechanisms of apathy are postulated to involve the brainstem and forebrain circuits that mediate goal-directed behavior. The functions of these circuits provide a model for understanding a provisional classification of apathy syndromes into cognitive, sensory, motor, and affective subtypes. This classification and its postulated neural basis has heuristic value for increasing our understanding of the disorders in which apathy is prominent.