Cognitive dysfunction is frequent in patients with multiple sclerosis (MS) and to date, there are no available treatments to improve cognition in this patient population. Some evidence from animal studies and small clinical trials suggest that aerobic exercise might beneficially affect cognitive function in MS. The aim of this randomized-controlled trial is to explore if an aerobic exercise training program can enhance cognition in MS. In addition, we will employ neuroimaging markers to determine if exercise alters measures of brain structure and function.