Multiple sclerosis (MS) is a demyelinating disease of the central nervous system that commonly leads to inflammatory and atrophic brain pathology, often causing cognitive impairment. MS-associated cognitive impairment was first described over a century ago. However, with the advent of standardized neuropsychological testing and quantitative brain imaging, the frequency, quality, and correlates of cognitive impairment are better understood. Dementia is rare in MS, although it is known to occur in 10 to 25% of patients. Our data suggest a frequency of 22% among clinic attendees. In addition to the cognitive impairments evident in MS dementia, changes in personality and social behavior also occur. For example, some patients develop euphoria sclerotica and marked deficiency in social empathy, conditions that in combination with executive dysfunction cause considerable hardship for patients and caregivers. These neuropsychiatric manifestations of MS dementia are correlated with magnetic resonance imaging indicators of brain atrophy, including ventricle enlargement, neocortical volume, and normalized whole brain volume. Recent developments in pharmacological treatment for disease progression and management of cognitive symptoms hold promise for patients suffering from the degenerative aspects of MS.