Physical activity has many health benefits; however, there has been concern that exercise may increase the risk of the development or progression of osteoarthritis (OA) of the knee. There is little doubt that injury increases the risk of OA, but the role of physical activity independent to injury is uncertain. Recently, magnetic resonance imaging has allowed an in-depth assessment of joints and relevant structural changes-this review covers the recent imaging data relevant to this area. In children and young adults, physical activity appears beneficial for knee cartilage, possibly even in structurally abnormal knees. In addition, there is consistent evidence showing aerobic and strengthening exercise improves OA symptoms later in life. However, there is limited evidence associating exercise with structural changes in later life and this lacks consistency, suggesting little or no effect. In the meantime, it appears safe to prescribe exercise in later life without major concern for structural deterioration, although caution is appropriate in those with bone marrow lesions until more information becomes available.