This article presents evidence for the reliability and construct validity of the Apathy Evaluation Scale (AES). Conceptually, apathy is defined as lack of motivation not attributable to diminished level of consciousness, cognitive impairment, or emotional distress. Operationally, the AES treats apathy as a psychological dimension defined by simultaneous deficits in the overt behavioral, cognitive, and emotional concomitants of goal-directed behavior. Three versions of the AES (clinician, informant, and self-rated) were evaluated for 123 subjects, ages 53-85, meeting research criteria for right or left hemisphere stroke, probable Alzheimer's disease, major depression, or well elderly control. Multiple forms of reliability (internal consistency, test-retest, and interrater) were satisfactory. Several types of validity evidence are presented for each version of the scale, including the following: ability of the AES to discriminate between groups according to mean levels of apathy, discriminability of apathy ratings from standard measures of depression and anxiety, convergent validity between the three versions of the scale, and predictive validity measures derived from observing subjects' play with novelty toys and videogames. Guidelines for the administration of the AES are presented, along with suggestions for potential applications of the scale to clinical and research questions.