Dementia encompasses both cognitive and noncognitive domains. Noncognitive symptoms include disorders of behavior, personality, mood, thought content, and perception and impaired functional ability. Behavioral, personality, mood, and thought content disorders have been labeled behavioral and psychological symptoms of dementia (BPSD) following a previous International Psychogeriatric Association consensus conference. Traditionally, BPSD have been poorly studied for several reasons, including the lack of importance attached to cognition in the diagnosis of dementia, the assumption that BPSD are secondary to cognitive and personality changes, the difficulty of accurately defining BPSD, and the paucity of standardized instruments to measure BPSD. Historically, all BPSD tended to be studied together under one broad umbrella and thus older data on individual BPSD are limited. Differing definitions, patient populations, study designs, data collection methods, measurement instruments, and methodologic issues influence the prevalence of the various BPSD reported in different studies, making comparisons between studies difficult.