Dementia of the Alzheimer type (DAT) is a disorder in the elderly, showing progressive decline of memory and various cognitive functions. There is no known effective treatment for DAT. Symptoms appear gradually and the course is steadily progressive. Neuropathologically, limbic and posterior association cortices of the cerebrum are preferentially affected. Abundant neuritic plaque, neurofibrillary tangles, and neuronal loss are detected in these areas. DAT is one of major disorders among senile dementia, frequent next to vascular dementia, in the Japanese. According to the age at onset, DAT has been classified into Alzheimer's disease (age at onset < or = 65) and senile dementia of Alzheimer type (age at onset >65). However, the rate of progression and clinical severity differ in these two groups, indicating that etiology of DAT is heterogeneous [correction of heterogenous]. Clinical diagnosis of DAT is mostly based on symptoms, mode of progression, and neuropsychiatric evaluation. DSM-IV has been recommended as a standard criteria. X-ray CT or MR imaging study can estimate atrophic alteration of DAT brain, and is of value in the differential diagnosis. In addition, PET or SPECT are useful tools for evaluating functional deficits of the cerebral cortex, since such abnormalities can be detected even from an early stage. Recent molecular genetic studies have disclosed that genetic background of DAT is heterogeneous [correction of heterogenous]. Not only mutation in a causative gene but also certain predisposing genes play roles in generating DAT pathology. However, it is still uncertain whether such genetic factors are correlated with the divergence of DAT phenotype.