Disturbances of mood, thought content, perception, and behavior are common concomitants of dementia that often produce suffering and excess disability. These disturbances also interfere with caregiving, precipitate institutionalization, and hasten death. The current lack of an effective means of preventing or controlling the pathophysiologic events that lead to dementia in late life has stimulated efforts to understand and treat these phenomena. In particular, clinically significant depression and psychosis often emerge in patients with dementia, regardless of its origins, and are an important focus of treatment.