MR imaging has had a significant impact on the understanding of multiple sclerosis. The procedure now plays an important role in initial diagnostic workup, replacing some other radiologic and paraclinical tests and often confirming clinically suggested locations of lesions. It also has contributed greatly to the understanding of the natural history of this disease, allowing objective assessment of disease load, detection of asymptomatic lesions, and differentiation between acute and chronic lesions. MR imaging is highly sensitive to inflammation and demyelination caused by multiple sclerosis, and although there is a long differential diagnosis for some of the MR findings, increasing experience has defined a number of relatively specific criteria for multiple sclerosis. Recent advances may allow faster imaging and highly objective lesion quantification, which will aid in therapeutic trials.