This article describes the pathophysiology of dementia and differentiates between cognitive and noncognitive symptoms that characterize this devastating illness. Relationships between brain anatomic and neurochemical systems and behavioral symptoms of dementia are discussed. An overview of the etiologies and neuropathologiesof dementia are presented as they relate to impairments in memory and intellectual abilities, personality changes, and behavioral symptoms. Recent genetic and molecular discoveries that have advanced our understanding of this complex spectrum of disorders and their treatment(s) are also highlighted.More than 70 diseases/disorders are associated with the progressive loss of memory and intellectual function known as dementia, although Alzheimer's disease (AD) is by far the most common cause. Current estimates suggest that more than four million Americans suffer from AD (Advisory Panel on Alzheimer's Disease, 1995). Although the diagnosis of dementia focuses on the clinical presentation of cognitive deficits, complex disturbances of behavior and emotion have been recognized as part of the disorder since Alois Alzheimer's original case report in 1907 describing the cerebral cortex and abnormal behaviors of a 55-yearold woman (Alzheimer, 1987). In contrast to the cognitive symptoms, which usually follow a welldescribed course of progressive decline, the occurrence and course of the noncognitive symptoms in dementia are less predictable (Tariot, 1994), developing at any stage during the disease process (Eisdorfer et al., 1992).There is a discrepancy between the labeling of symptoms by health care professionals and complaints by family members about their loved one's behavior. For example, dementia is generally defined by clinicians as progressive deterioration of cognitive abilities, however families of dementia patients often define the stages of illness with behavioral (noncognitive) markers. In fact, behavioral complications are a leading cause of institutionalization in persons with dementia (Knopman, Kitto, Deinard, & Heiring, 1988;Steele, Rovner, Chase, & Folstein, 1990).Disease progression is not measured simply by declining scores on the Mini-Mental State Exam or other clinical rating scales, but rather includes such symptoms as the amount of sleeplessness at night, the person's degree of agitation or psychosis, and the level of depressed mood. A key unanswered scientific question is how central these noncognitive behavioral symptoms are to