Stem cell therapy is a promising new treatment option for a range of conditions including osteoarthritis (OA). Recent clinical studies in patients with knee OA have shown that stem cell therapy may improve pain scores and joint mobility for up to 12-24 months. However, the studies vary in study design and quality making comparisons difficult, and there is a lack of controls. The mode-of-action of stem cells in the joint is still largely unknown. A large number of clinical studies are underway and will increase our understanding of the efficacy and function of stem cells in OA.