What is the nature and the neural substrate of mental representation? This paper reviews findings from experimental psychology demonstrating that visual imagery and perception have similar characteristics. These results suggest that visual imagery and visual perception rely on the same neural substrate. Brain imaging studies as well as clinical observations of neurological patients support this hypothesis. Visual imagery involves visual cortical areas. However, selective visual impairments following damage to the cortical visual system may produce some dissociation between imagery and perception. Similar observations concerning motor imagery are now established in both normals and in neurological patients. Evidence that motor imagery and motor control share some modality specific neural representations are clearly supported by tomographic measurements of cerebral blood flow.