Multiple Sclerosis (MS) is a chronic, disabling neurologic disease characterized by damage to myelin and axons in the central nervous system (CNS). Current therapy for MS primarily consists of immunomodulatory drugs aimed at preventing future CNS injury; no treatments are currently available to repair existing damage, and symptomatic treatments to improve neurological function are quite limited. Rehabilitation approaches, such as exercise, have long been a staple of MS therapy. Exercise therapy in MS patients has proven benefits on mobility, mood, motor function and quality of life. While the beneficial effects of exercise on cardiovascular and musculoskeletal function are well known, there has recently been increased focus on the positive effects of exercise on brain structure and function. The overall aim of this proposal is to determine whether exercise can promote beneficial changes in brain function in MS patients. The investigator's central hypothesis is that cardiovascular exercise alters both brain structure and brain function in MS patients, and that these changes can be identified and monitored via imaging techniques that evaluate regional brain volumes and functional connectivity. When brain activity measured at one area fluctuates in a coherent manner with that recorded in a different area, those brain regions are considered to be functionally connected. Using resting-state magnetoencephalography (MEG), the investigator's laboratory has demonstrated that these patterns of correlated brain activity are abnormal in MS patients.