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The N400 component's amplitude is standardly reduced for predictable words. But it is not clear whether this reduction truly reflects preactivation of the critical word or whether it just indexes the difficulty of integrating the word with the sentence. To adjudicate between these two accounts, event-related potentials were recorded while participants read short stories. In half of the stories, just before the story-final sentence, we induced an explicit prediction of a congruent or an incongruent target word, thereby preactivating this word. Inducing prediction for the incongruent target word eliminated the N400, fully supporting the preactivation theory. This implies that the N400 is a marker of prediction and that prediction is an essential aspect of sentence comprehension. In addition, an analysis of the ERPs on the words preceding the critical target word provides evidence on how the target word becomes preactivated by the context.