Current treatments of osteoarthritis (OA) focus on pain and loss of joint function. When these interventions fail, the destroyed joint is replaced by implants of metal, plastic and ceramics. In the future, we need to detect cartilage loss before it is too severe, prevent further loss and stimulate regrowth of lost cartilage. Research in tissue engineering can help us understand the complex requirements for regeneration of joint cartilage. Results from animal experiments and small, uncontrolled, open series of human cartilage repair suggest that functional repair can be accomplished in some joints in some patients. However, outcome is inconsistent. Do we need to recreate the original hyaline joint cartilage or will something else work as well? It is far from clear what factors determine a successful repair or what method is best. The durability of repair tissue is uncertain. The cost-benefit equation is unresolved, and current surgical interventions are associated with significant cost and morbidity. What is the 'number-needed-to-treat' to prevent one knee/patient lost to early retirement or future OA? The outcome measures used to determine success or failure of the repair deal with cartilage, joint and patient. The relationship between these outcome dimensions is unclear. However, the outcome as judged by the patient using standardized measures is the gold standard.