Magnetic resonance imaging (MRI) has been shown to be a good method of visualizing the lesions in MS. We have studied several applications of MRI to the evaluation of patients and experimental models. In diagnosis, MRI is the most sensitive test for the demonstration of dissemination of lesions in space. Pathological correlation studies show that MRI reliably measures the extent of chronic demyelination. Experimental studies show that MRI detects acute inflammatory lesions and measures their evolution. MRI also is a reliable measure of the extent of the MS process, serial MRI scans detect evidence of disease activity in MS not always disclosed by clinical evaluation. MRI will have an enormous future impact on the evaluation of patients in clinical studies and in understanding the evolution of pathological processes.