In less than a decade, Magnetic Resonance Imaging (MRI) has become the examination of choice in patients with suspected multiple sclerosis (MS). It is the best paraclinical test in assessing dissemination in space of lesions. With serial MRI scans, even dissemination in time can be detected. Using serial Gd-DTPA-enhanced MRI scans, the evolution of lesion can be easily followed. MRI studies in MS patients have contributed to shape current ideas about the pathogenesis of the disease showing that focal breakdown of the blood-brain barrier (BBB) is an early event, if not the first, in the evolution of MS lesions. A number of asymptomatic lesions can be detected by MRI in MS patients, suggesting an ongoing disease activity independent of the clinical appearance. Thus, besides its diagnostic usefulness, MRI will represent the best tool to evaluate effectiveness of treatments in therapeutical trials in MS.