Multiple sclerosis (MS) is the most common demyelinating disease, in which the insulating covers of nerve cells in the brain and spinal cord are damaged. This damage disrupts the ability of parts of the nervous system to transmit signals, resulting in a range of signs and symptoms, including physical, mental, and sometimes psychiatric problems. Specific symptoms can include double vision, blindness in one eye, muscle weakness, and trouble with sensation or coordination. MS takes several forms, with new symptoms either occurring in isolated attacks (relapsing forms) or building up over time (progressive forms). In the relapsing forms of MS, between attacks, symptoms may disappear completely, although some permanent neurological problems often remain, especially as the disease advances.

While the cause is unclear, the underlying mechanism is thought to be either destruction by the immune system or failure of the myelin-producing cells. Proposed causes for this include genetics and environmental factors, such as viral infections. MS is usually diagnosed based on the presenting signs and symptoms and the results of supporting medical tests.

No cure for multiple sclerosis is known. Treatments attempt to improve function after an attack and prevent new attacks. Physical therapy and occupational therapy can help with people's ability to function. Many people pursue alternative treatments, despite a lack of evidence of benefit. The long-term outcome is difficult to predict; better outcomes are more often seen in women, those who develop the disease early in life, those with a relapsing course, and those who initially experienced few attacks.

Multiple sclerosis is the most common immune-mediated disorder affecting the central nervous system. Nearly one million people have MS in the United States in 2022, and in 2020, about 2.8 million people were affected globally, with rates varying widely in different regions and among different populations. The disease usually begins between the ages of 20 and 50 and is twice as common in women as in men. MS was first described in 1868 by French neurologist Jean-Martin Charcot. The name "multiple sclerosis" is short for multiple cerebro-spinal sclerosis, which refers to the numerous glial scars (or sclerae – essentially plaques or lesions) that develop on the white matter of the brain and spinal cord.