Loss of cognitive function in the elderly population is a common condition encountered in general medical practice. Diagnostic criteria and approaches have become more refined and explicit in the past several years. Precise diagnosis is feasible clinically. In this article, the precursor state and major subtypes of dementia are considered. Mild cognitive impairment is the term given to patients with cognitive impairment that is detectable by clinical criteria but does not produce impairment in daily functioning. When daily functioning is impaired as a result of cognitive decline, dementia is the appropriate syndromic label. Specific causes of dementia tend to have distinctive clinical presentations: the anterograde amnesic syndrome of Alzheimer disease; the syndrome of dementia with cerebrovascular disease; the syndrome of Lewy body dementia with its distinctive constellation of extrapyramidal features, disordered arousal, and dementia; the behavioral-cognitive syndrome of frontotemporal dementia; the primary progressive aphasias; and the rapidly progressive dementias. Because dementia syndromes have distinctive natural histories, precise diagnosis leads to a better understanding of prognosis. As new treatments become available for Alzheimer disease, the most common of the dementias, accurate diagnosis allows the appropriate patients to receive treatment.