Multiple sclerosis (MS) is a chronic autoimmune disease of the central nervous system characterized by exacerbations of neurological dysfunction due to inflammatory demyelination. Neurologic symptoms typically present in young adulthood and vary based on the site of inflammation, although weakness, sensory impairment, brainstem dysfunction, and vision loss are common. MS occurs more frequently in women and its development is complex—genetics, hormones, geography, vitamin D, and viral exposure all play roles. Early MS is characterized by relapsing-remitting course and inflammation of the white matter, although as patients age, the disease often transitions to a pathologically distinct secondary progressive phase with gradual disability accrual affecting gait, coordination, and bladder function. A minority of patients (10%) have disease that is progressive at onset. In the past decade, there has been a remarkable expansion in disease-modifying therapy for MS, but treatment of progressive disease remains a challenge. This article reviews foundational concepts in MS and emerging work that has reshaped understanding of the disease, providing new insight for therapeutic advance.