Imaging is crucial for the diagnosis and differential diagnosis of arthritis. Plain radiography is the primary tool used to assess and monitor the progression of arthritis or its response to therapy. The newer imaging modalities offer a more complete assessment of the joints in health and disease. Magnetic resonance (MR) imaging with gadolinium administration allows direct visualization of the inflamed synovium and pannus. With MR imaging articular cartilage can be seen as well as joint effusion and hemosiderin, if present. Small (and large) joint effusions are readily revealed using ultrasonography, which is a relatively easy and inexpensive technique.