Degenerative diseases of the basal ganglia, such as Huntington's disease (HD), Parkinson's disease, and Wilson's disease, are characterized by motor, cognitive, and psychiatric manifestations. HD, in particular, can be considered a paradigmatic neuropsychiatric disorder that has all three components of the "Triadic Syndromes": dyskinesia, dementia, and depression. The authors examine the phenomenology, prevalence, and management of psychiatric disturbances occurring in diseases of the basal ganglia. They address psychiatric conditions such as depression, mania, psychosis, obsessive-compulsive disorders, aggression, irritability, apathy, sexual disorders, and delirium, discussing subtleties of diagnosis, and making reference to more unusual disorders of the basal ganglia, such as postencephalitic parkinsonism and Fahr's disease.