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 neuro-pathologies of 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.