Mild cognitive impairment (MCI) has become an important topic for clinical practice and research. MCI refers to a clinical transitional state between the cognitive changes of aging and the earliest clinical features of dementia. Originally, the construct referred to a memory impairment in the setting of preserved nonmemory cognitive performance and functional abilities, but more recently the term has been expanded to include other cognitive domains besides memory. Most of the literature refers to the amnestic form of MCI, which is likely a precursor of clinical Alzheimer's disease. Much research generated in the past decade on MCI indicates that the criteria are available and reliable and that the outcomes of patients are known. Some predictors of progression have been identified, and others are being evaluated. Neuroimaging studies document the intermediate state of brain structural and functional features in patients with MCI, while neuropathologic data confirm transitional pathologic findings. Several randomized clinical trials have been completed recently, and although most are essentially negative, one trial suggested that donepezil might be effective for the treatment of MCI for a limited period of time. Implications of MCI for clinical practice and future research directions are discussed.