Exposure to electromagnetic energy has potent signalling effects upon articular cells including chondrocytes, synoviocytes and osteoblasts. Attention has focused on two actions – the altered synthesis of cytokines and enzymes, and the enhanced synthesis of bone and cartilage extracellular matrix (ECM) molecules. In vitro studies with human and bovine articular cartilage have shown increased aggregcan synthesis, glycosaminoglycan content, and biomechanical aggregate modulus with EMF exposure. Osteoarthritic (OA) cartilage responds similarly depending upon the severity of the OA with early OA cartilage responding more robustly. On these bases, two in vivo studies have been done with the Dunkin-Hartley guinea pig model of spontaneous OA. Both studies demonstrated preservation of ECM with increased aggrecan synthesis, matrix glycosaminoglycan and type 2 collagen content, and reduced histological-histochemical (Mankin) scores. Suppression of matrix metalloproteases and IL-1, together with increased TGFb were also observed. Responses to various EMF configurations, in terms of amplitude, frequency, and exposure duration have been described, indicating dose responsiveness. These studies suggest the conclusion that exposure to specific EMFs reduce the progression of early OA. A randomized clinical trial is underway. EMFs may be a disease-modifying therapy for OA, resulting in maintenance of ECM and improvement in the cytokine environment of OA joints.