Slowing of radiographic joint space narrowing represents the only recommended imaging-based outcome measure to assess structural disease progression in osteoarthritis (OA) clinical trials. There are no effective disease-modifying OA drugs. The ability of magnetic resonance (MR) to image structures within the knee and to visualize cartilage morphology and composition gives MR imaging a critical role in understanding the natural history of the disease and in the search for therapies. In this article, the roles and limitations of conventional radiography and MR imaging, focusing on knee OA, and the use of other modalities in clinical practice and OA research are described.