The N400 is an event-relate potential (ERP) component typically elicited by unexpected linguistic stimuli. P600 is another such ERP. N400 is characterized as a negative deflection (topologically distributed over central-parietal sites on the scalp), peaking approximately 400ms (300-500ms) after the presentation of the stimulus. In models of speech comprehension, N400 is often associated with the semantic integration of words in sentence context; its finding is interpreted as pointing to the activation of a process working on semantics in the general time frame. This article deals with the effect in itself as well as with the interpretations it has earned in psycholinguistic/neurolinguistic research.