This paper discusses the definition of apathy, reviews its differential diagnosis, and proposes a classification for the conditions that may produce it. Apathy is defined as diminished motivation not attributable to diminished level of consciousness, cognitive impairment, or emotional distress. In its differential diagnosis, abulia, akinesia and akinetic mutism, depression, dementia, delirium, despair, and demoralization must be ruled out. Classification of apathy is organized in terms of its adaptive and functional consequences, its relationship to personality or to sociocultural or environmental events, and its association with psychiatric, neurological, and medical disorders. An approach to assessment and treatment is proposed.