There are two types of dementia with early onset: (i) presenile dementias; and (ii) senile dementias with early onset. Most patients who develop dementia before 65 years of age have Alzheimer's disease (AD). The remainder are likely to have vascular dementia (VaD), frontotemporal dementia, head injury, alcohol intoxication, or metabolic disorder. Presenile dementias, caused by frontotemporal lobar degeneration, progressive supranuclear palsy, and corticobasal degeneration, usually occur in patients of presenile and are rarely seen in patients of senile age. Although the factors responsible for the accelarted onset of the illness are not fully known, genetic abnormalities appear to be important in some types of presenile dementia, such as frontotemporal dementia with parkinsonism linked to chromosome 17. Conversely, senile dementias such as sporadic AD and VaD commonly occur in patients of senile age. These disorders may also occur in patients of presenile age, although less frequently. Alzheimer's disease was originally classified as a ‘presenile dementia’. Since the 1980s, ‘senile dementia of Alzheimer type’ (SDAT) and ‘Alzheimer's disease’ have been considered to belong to the same pathological entity and both are now known as ‘dementia of Alzheimer's type (DAT)’ or merely ‘Alzheimer's disease’. Rapid progression of cognitive impairment with neuropsychological syndromes and neurological symptoms has been considered a characteristic of early onset AD. However, recently, neurological symptoms such as spastic paraparesis, seizures, and myoclonic convulsions have been reported to occur infrequently in early onset AD, although language problems and visuospatial dysfunctions are common. There are at least three dominant genes that have been identified in cases of familial Alzheimer's disease with early onset, namely the amyloid precursor gene (APP), and the genes encoding presenilin 1 (PSEN1) and presenilin 2 (PSEN2). Therefore, genetic abnormalities are important factors contributing to the earlier onset of the illness. It is also important to investigate the pathophysiological mechanism in relation to genetic abnormalities, environmental factors, physical illnesses, and metabolic disturbances to understand the processes underlying the development of dementia with early onset.