Mild cognitive impairment (MCI) is a clinical diagnosis based on subjective cognitive decline, objective cognitive impairment, and relative preservation of activities of daily living. The diagnosis may be established via clinical interview, collateral history from an informant, and psychometric examination. Various consensus groups have proposed criteria for MCI in Alzheimer's disease (AD), Parkinson's disease, dementia with Lewy bodies, and vascular cognitive impairment. These diagnostic criteria have subtle but important differences. Criteria for subjective decline vary according to whether memory is impaired or whether impairment in any cognitive domain is sufficient. There are also differences with respect to whether the subjective decline is noted by the patient, a carer, or a clinician. The precise thresholds for classifying objective cognitive impairment also vary between various diagnostic criteria. There are also differences in the description of functional abilities. Once established, the diagnosis of MCI may be refined to 1 of 4 subtypes based on the pattern of cognitive impairment. The 4 subtypes are defined according to whether or not memory is impaired and whether 1 or more cognitive domains are impaired. Once a diagnosis of MCI has been made, the patient and the family should be counseled about social and legal implications as well as strategies for reducing the risk of progression to dementia. The main utilities of MCI as a nosology are to understand the natural history of neurodegenerative disorders such as AD, to identify those at increased risk of progressing to develop dementia, and to identifying individuals for putative treatments.