Early-onset dementia, presenting before age 65 years, is increasingly recognized. It is often difficult to diagnose, since non-Alzheimer's etiologies and unusual dementias are common. These conditions are more commonly genetic, and important potentially inherited causes of early-onset dementia include early-onset Alzheimer's disease, frontotemporal dementia, Kufs' disease, and Niemann-Pick disease type C. For each of these diseases, this review provides information on common clinical presentations, etiology, pathophysiology, and current and experimental treatments. A discussion of the diagnosis and workup for early-onset dementia is included with an emphasis on conditions that may have other involved family members.