Dementia, also called major neurocognitive disorder, is characterized by a chronic progressive loss of cognitive function in the absence of fluctuating consciousness. It represents a primarily geriatric syndrome that may be caused by one of several underlying conditions. There is insufficient evidence to support universal screening for cognitive impairment in older adults; however, clinicians should be alert to patient and caregiver concerns about cognitive changes and investigate such concerns with validated cognitive assessment tools. Alzheimer disease is the leading cause and prototypical form of dementia, presenting insidiously and causing progressive cognitive impairment with increasing severity over a period of years. Vascular dementia is the second most common form of dementia and often co-occurs with other progressive cognitive disorders. Lewy body dementias encompass Parkinson disease dementia and dementia with Lewy bodies, which have similar features and are differentiated primarily by the order of motor and cognitive symptom onset. Frontotemporal dementias occur earlier than other forms of dementia, progress rapidly, and often have a genetic component. An understanding of the conditions that cause dementia will assist clinicians in making an accurate diagnosis and providing appropriate treatment recommendations and counseling regarding the diagnosis and prognosis.