The early detection of young people at-risk of developing a severe mental illness like schizophrenia offers the opportunity of introducing treatment earlier than currently possible. There is some evidence that early intervention improves prognosis and functional outcome, or even prevents the full clinical manifestation of the condition in some individuals. A key prerequisite to facilitate early intervention would be a biomarker that can reliably predict a transition to schizophrenia. A smaller event-related mismatch negativity (MMN) potential has emerged as one of the most robust psychophysiological finding in schizophrenia akin of a biomarker of the condition. More recent research further demonstrates that MMN, but also P3a amplitudes, are already reduced in the prodromal phase of illness. Several lines of pre-clinical and clinical research support this notion and are reviewed in this article together with current obstacles, which are still limiting the translation of MMN as a biomarker into clinical practice.