Prevalence estimates of cognitive impairment in multiple sclerosis (MS) range from 40% to 65%, depending on the research setting. Cognitive dysfunction virtually encompasses all the disease stages and types of clinical course, although it is generally less frequent in relapsing-remitting (RR) patients compared with secondary progressive (SP) patients, and tends to be less frequent in primary progressive (PP) patients. Moreover, it causes role limitations in work and social life, independently of the degree of physical disability. Relatively little is known about the evolution of cognitive impairment in MS, particularly starting from the early stages of the disease. Controlled studies, however, have clearly shown that cognitive deterioration tends to progress over time. Among clinical predictors, incipient cognitive decline seems to be the major risk factor for further deterioration in the short-term. In the long-term, the likelihood increases that also patients with initial cognitive preservation may deteriorate. As for magnetic resonance imaging (MRI), there are consistent, albeit moderate, correlations between the progression of cognitive impairment and increasing brain lesion load and brain atrophy. The aim of this paper is to provide a review of existing cross-sectional and longitudinal studies on cognitive deterioration in MS.