Many patients with multiple sclerosis exhibit behavioural changes, including alterations cognitive functions and psychiatric abnormalities. Increasingly sophisticated neuropsychologic assessment techniques plus magnetic resonance imaging have added to our knowledge in this area, showing that these alterations may be more common than previously recognized. Research studies have attempted to address the question of whether these changes are primarily function of the demyelinating process itself or a reaction to the disability produced by the disorder. This is an important issue in terms of possible intervention. This paper will include a review of the literature.