Infrequent ("deviant") sounds occurring in a sequence of repetitive ("standard") sounds elicit an event-related brain potential (ERP) response called the mismatch negativity (MMN) even in the absence of attention to these sounds. MMN appears to be caused by a neuronal mismatch between the deviant auditory input and a sensory-memory trace representing the standard stimuli. This automatic mismatch process has presumably a central role in discrimination of changes in the acoustic environment outside the focus of attention. Thus, localizing cerebral generators of MMN might help identify brain mechanisms of auditory sensory memory and involuntary attention. This review summarizes results from studies aimed at localizing MMN generators on the basis of (1) scalp-distribution, (2) magnetoencephalographic (MEG), (3) intracranial, and (4) brain-lesion data. These studies indicate that a major MMN source is located in the auditory cortex. However, the exact location of this MMN generator appears to depend on which feature of a sound is changed (e.g., frequency, intensity, or duration), as well as on the complexity of the sound (e.g., a simple tone versus complex sound). Consequently, memory traces for different acoustic features, as well as for sounds of different complexity, might be located in different regions of auditory cortex. However, MMN appears to have generators in other brain structures, too. There is some evidence for contribution of frontal-lobe activity to the MMN, which might be related to the involuntary switching of attention to a stimulus change occurring outside the focus of attention. In addition, intracranial MMN recordings in animals suggest that at least in some species, MMN subcomponents also may be generated in the thalamus and hippocampus.