Early-onset dementia (EOD, <65 years at onset) is a relatively common and frequently misdiagnosed condition. One reason for misdiagnosis is that EOD has a more varied differential diagnosis than late-onset dementia (LOD). For example, Alzheimer's disease (AD), the preponderant LOD, makes up only about one-third of EODs; the rest are due to vascular dementias, frontotemporal lobar degenerations, traumatic head injury, alcohol-related dementia, and a great many other conditions. Another reason for misdiagnosis is that early-onset AD may have predominant cognitive deficits other than memory loss and a potential familial inheritance with spastic paraparesis, seizures, or myoclonus. A third reason is that EOD often presents with neuropsychiatric features out-of-proportion to any cognitive deficits. Despite these obstacles, it is important to accurately diagnose EODs, particularly because they differ in management and course. Clinicians can successfully diagnose most EODs with careful cognitive and family histories, mental status and neurological examinations, and neuroimaging.