Multiple sclerosis (MS) is a relatively common, chronic progressive neurological illness affecting individuals primarily in the third and fourth decades of life. Autopsy studies indicate that the disease preferentially attacks white matter throughout the CNS, including the cerebral hemispheres. This article reviews the current state of knowledge regarding cognitive dysfunction in MS and relates these findings to neuropathological changes. The view that affective disturbance may also result from cerebral demyelination is presented, along with a brief discussion of MS as a prototype "subcortical" dementia. Finally, methodological problems intrinsic to the study of MS are presented, and suggestions for future research are made.