Many new therapeutic strategies have been and are being developed to prevent, correct, or slow the progression of osteoarthritis. Our ability to evaluate the efficacy of these techniques, or to determine the situations for which they might provide the most benefit, critically depends on diagnostic measures that can serve as proxies for the present or predicted state of the cartilage. We focus here on a body of work surrounding the development of magnetic resonance imaging (MRI) techniques to noninvasively image the glycosaminoglycan (GAG) concentration of articular cartilage. These techniques are based on the concept of fixed charge in cartilage resulting from the glycosaminoglycans. Starting with sodium MRI, and the subsequent development of delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) based on proton MRI, these techniques permit "visualization" of the charged GAG distribution in cartilage in vitro or in vivo. The dGEMRIC technique has been used in preliminary clinical studies to understand treatment strategies and to monitor disease, and as such is allowing studies that a decade ago would have been impossible. This new technical capability offers the promises of speeding development of effective therapies and focusing their use in areas where they can be most successful.