Structural brain imaging with CT or MRI should be performed in any patient with progressive cognitive impairment to rule out a reversible cause, such as a benign brain tumor, a subdural hematoma, or hydrocephalus. But imaging can also help separate the various types of degenerative dementia and facilitates the prognosis of these disorders. Medial temporal atrophy is an early finding in Alzheimer disease (AD). Early in AD, metabolism is decreased in the parietotemporal association cortex and retrosplenial region. When these imaging findings are present in mild cognitive impairment, it is more likely to evolve to AD. Amyloid deposition in AD and Lewy body dementia can now be imaged, differentiating these disorders from dementias without amyloid deposition, such as frontotemporal dementia and corticobasal degeneration. Atrophy or the decrease in brain metabolic activity associated with the dementias could be used as surrogate markers of response to new therapies in clinical trials.