Multiple sclerosis is an immune–mediated inflammatory demyelinating disease of the central nervous system clinically characterized by relapses and remissions of neurological disturbance. A typical relapse, exemplified by optic neuritis, increases in severity over a week or two and after approximately one month begins to remit. Resolution takes place over the course of two to three months. In the early stages, clinical recovery is virtually complete, though persistent abnormalities of conduction can usually be detected by evoked potential techniques and persistent structural abnormalities can be detected by magnetic resonance imaging (MRI). These techniques, together with cerebrospinal fluid examination for oligoclonal IgG, provide supporting evidence for the diagnosis which, in the absence of a specific test, nevertheless remains primarily clinical. The course of the disease is very variable, but after a number of years neurological deficit begins to accumulate after each relapse. In most patients, the relapsing and remitting phase of the disease is followed by a phase of continuous progression of disability. Cognitive disturbances can be detected in many patients even quite early in the course of the illness. Deficits in attention, memory and executive skills may be prominent and tend to become increasingly prominent as neurological deficit increases, although this is not always the case. There is some correlation between the extent of MRI abnormalities in the cerebral white matter and the severity of cognitive deficit. Depression and anxiety are commonly experienced but are poorly correlated to the lesion load seen on MRI. In contrast, the much rarer psychotic symptoms, euphoria and emotional lability are closely linked to the severity of white matter disease.