MR imaging is becoming increasingly important in the evaluation of multiple sclerosis based on its sensitivity to acute, often subclinical events in the brain and because it provides a basis for measuring the accumulation of disease over time. Contrast-enhanced MR imaging in particular evaluates disease at the fundamental level of events affecting the blood-brain barrier. This review emphasizes (a) recent developments in the use of contrast-enhanced MR imaging as a measure of disease in patient groups and individuals and (b) its emerging role in evaluating new therapies.