Multiple sclerosis (MS) is a 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). Between attacks, symptoms may disappear completely; however, permanent neurological problems often remain, especially with the advancement of the disease.

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 being triggered by a viral infection. MS is usually diagnosed based on the presenting signs and symptoms and the results of supporting medical tests. There is no known cure for multiple sclerosis. Treatments attempt to improve function after an attack and prevent new attacks. Medications used to treat MS, while modestly effective, can have side effects and be poorly tolerated. Physical 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, with good outcomes 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. Life expectancy is on average 5 to 10 years lower than that of the unaffected population.

Multiple sclerosis is the most common immune-mediated disorder affecting the central nervous system. In 2015, about 2.3 million people were affected globally, with rates varying widely in different regions and among different populations. In that year, about 18,900 people died from MS, up from 12,000 in 1990. 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 refers to the numerous glial scars (or sclerae – essentially plaques or lesions) that develop on the white matter of the brain and spinal cord. A number of new treatments and diagnostic methods are under development.