Recently, many studies classify nondemented persons into groups that represent a stage between normal aging and dementia, i.e. a high-risk state for developing a dementia disorder. There is a need to identify subjects in the preclinical stages of AD in order to benefit from possible pharmaceutical developments of future drugs that might delay or the onset of dementia. The literature contains many efforts to categorise people according to various symptoms and signs in order to accurately identify people at high risk of future development of AD. Although the various criteria differ somewhat, there are essentially two common features: 1) the person does not fulfil criteria for dementia and 2) they exhibit some form of cognitive impairment. The various signs and symptoms of cognitive impairment will be discussed in the next section, taking into account different methods and tools to define cognitive impairment.

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