Dementia is defined as acquired impairments of memory and other cognitive domains and failure in activities of daily living. The prevalence of dementia increases continuously with age, which is 5% of the over 65 years and 20% of the over 80 years. There are many causes of dementia, but Alzheimer’s disease (AD) and vascular dementia account for over 80% of cases. The diagnosis of AD is primarily based on the clinical definition of the dementia syndrome with exclusion of other systematic illness or space occupying lesion of CNS. Definitive cerebral lesion such as cerebral infarct or hemorrhage is essential for the diagnosis of vascular dementia. Therefore, neuroimaging, which is recommended by international guidelines, serves an important supportive function in the final establishment of a clinical diagnosis of AD and vascular dementia. This section will outlined structural, functional and molecular neuroimaging including brain CT and MRI, SPECT, PET, and multimodality MRI in the diagnosis and research of AD and vascular dementia.