Psychiatric symptoms and behavioral disturbances are of importance in dementia of the Alzheimer type (DAT) but have been relatively neglected by researchers. The main psychiatric symptoms are disorders of thought content (delusions and paranoid ideation), disorders of perception (hallucinations and misidentifications) and disorders of affect (depressed and elevated mood). They occur commonly in DAT, and there is evidence that they may indicate subtypes of the disorder in view of their demographic, structural, and prognostic correlates. Personality changes are very common, and these may be the earliest manifestation of dementia. Behavioral disturbances tend to be associated with advanced dementia and are particularly disruptive to caregivers. Structural correlates suggest that some behaviors are a direct result of brain damage. Psychiatric and behavioral features in DAT are important, as they may serve as a model for symptomatology in functional psychiatric disorders.