Early diagnosis, followed by early initiation and optimal adjustments of aggressive therapies, are acknowledged as essential to optimize long-term clinical and radiological outcome in rheumatoid arthritis (RA). This requires sensitive methods for detection and monitoring of the primary feature of RA--the synovitis. In comparison with conventional methods, magnetic resonance imaging (MRI) offers assessment of the rheumatoid synovium with improved sensitivity to early pathology and to change. Various aspects such as volume, vascularity, and edema can be assessed by different metrological approaches. MRI findings are of prognostic value to the long-term radiological outcome. This article reviews current knowledge on MRI for assessment of the synovium in RA, focusing on the validity of MRI measures of synovitis. Future perspectives and suggested research priorities are described. The rationale is provided for MRI becoming the new gold standard for assessment of RA joints and for MRI assessments of synovitis being reliable and valid measures of rheumatoid disease activity.