Vitamin D deficiency is very common in older people and has been linked with osteoporosis and falls in both older women and men. Emerging data suggests that it also plays an important role in the pathogenesis of knee OA. Firstly, vitamin D may have direct effects on chondrocytes in osteoarthritic cartilage; secondly, chronic vitamin D inadequacy in adults has adverse effects on calcium metabolism, osteoblast activity, matrix ossification and bone density, and thus could impair the ability of bone to respond optimally to pathophysiological processes in OA; and thirdly, low vitamin D levels are associated with loss of muscle strength and muscle mass in older men and women, which may be associated with an increased risk of knee OA. Some observational studies have shown that vitamin D insufficiency is associated with the progression and development of radiographic knee or hip OA. Recently we have demonstrated that baseline serum levels of 25-hydroxy-vitamin D(25-(OH)D) predicts change in cartilage volume in older adults over 2 years, and increases in vitamin D levels are associated with a further protective association. This suggests that vitamin D supplementation may enhance cartilage and bone health, and thus prevent disease progression in patients with knee OA.