This article addresses two issues about the neural bases of mental imagery. The first issue concerns the modality-specificity of mental images, that is, whether or not they involve activity in visual areas of the brain. The second issue concerns hemispheric specialization for the generation of mental images. We compared event-related potentials recorded under two conditions: one in which subjects were shown words and asked to read them and one in which subjects were shown words and asked to read them and generate visual mental images of the words' referents. Imagery caused a slow, late positivity, maximal at the occipital and posterior temporal regions of the scalp, relative to the comparison condition, and consistent with the involvement of modality-specific visual cortex in mental imagery. Also noted was an asymmetry in the imagery-related ERP, consistent with left-hemisphere specialization for mental image generation. Similar results were obtained when subjects listened to auditorily presented words with and without instructions to generate mental images. To assess the specificity of the relation between these ERP effects and mental imagery, we compared the ERP changes brought about by imaging with those brought about by another effortful task using the same stimulus words: proofreading the words for occasional misspellings. This produced changes that differed in polarity, time course, and scalp distribution from the imagery-related changes.