In around 85% of cases, multiple sclerosis (MS) starts with an acute neurological episode, a clinically isolated syndrome, which is considered to be the first clinical episode of relapsing-remitting MS (RRMS). It is characterised by the presence of acute relapses, after which there is normally good functional recovery. About 15-20 years after symptom onset, most patients develop secondary progressive MS, characterised by a gradual and irreversible neurological decline. MS affects women more frequently than men, with a ratio of 2-3:1. In RRMS the disease generally starts in the late 20s. Symptom onset in those with primary progressive MS occurs around the age of 40. Clinical relapses in RRMS reflect acute inflammation in the CNS, resulting in specific clinical syndromes including optic neuritis, myelitis, brainstem/cerebellum syndrome and supratentorial syndrome. In progressive MS, progression generally presents with gradual loss of power in the lower limbs, which may be asymmetric. It is often accompanied by sensory disturbances in the lower limbs, bladder and bowel symptoms. Investigations need to rule out conditions that can mimic an inflammatory-demyelinating disease of the CNS and determine the presence of dissemination in space and dissemination in time of the inflammatory-demyelinating disease. There is no confirmatory test for MS and it remains essentially a clinical diagnosis.