ObjectiveThis article reviews research on the main characteristics of mismatch negativity (MMN) and its applications in neuropsychiatry.BackgroundEvent-related potentials (ERPs) have been used to study many aspects of information processing. Mismatch negativity is an early auditory ERP that has been identified as an index of an automatic (preconscious) alerting mechanism stimulating an individual to attend to unexpected environmental events. Disturbances of MMN may relate to abnormalities of auditory information processing contributing to the pathophysiology of neuropsychiatric conditions.MethodThe authors review (1) studies that have evaluated the electrophysiological aspects of MMN and (2) studies that have investigated the different applications of MMN in neuropsychiatry.ResultsThe first part of this article describes the characteristics of MMN, its cerebral origins, and electrophysiological parameters. We then discuss the role of “echoic memory” as well as that of attention and vigilance. In the second part of the article, disturbances in MMN associated with schizophrenia, depressive illness, dementing processes, and other neuropsychiatric states are discussed.ConclusionsMMN is a preconscious cognitive ERP, the main generators and functions of which are well defined. Observations relating to the origins of MMN and its role in early auditory information processing together with its possible behavioral significance, combined with observations of MMN aberrations in psychiatric conditions, may provide novel insights into the pathophysiology of neuropsychiatric states.