Magnetic resonance imaging (MRI) is very sensitive in depicting multiple sclerosis (MS) lesions, but its specificity is poor. New sequences such as fast spin echo and FLAIR (fluid attenuated inversion recovery) improve the detection of lesions. The exploration of the whole central nervous system, brain, optic nerves and spinal cord improves sensitivity and specificity. The existence of lesions at different ages responds to temporal dissemination. MRI has also allowed to better understand the natural history of MS, showing 5 to 10 times more radiological than clinical activity. In case of isolated demyelinating syndrome, MRI is the best predictor of the occurrence of definite MS and of the severity of disability in the subsequent 10 years. However, the diagnosis of MS remains clinical, and systematic control MRI are not useful in clinical practice.