Molecules derived from cartilage are present in body fluids where they act as markers of the degradation or synthesis of specific components of the extracellular matrix of this shock-absorbing tissue. In joint fluid, the level of a marker molecule can be used to detect changes taking place in the cartilaginous tissues within that joint. In blood or urine, on the other hand, the level can be used to identify systemic changes or abnormalities affecting most or all the cartilages in the body. These specific markers of metabolic processes already have proved useful in identifying increased catabolic as well as anabolic activities in articular cartilage following traumatic knee joint injury. The markers are potentially useful for monitoring disease activity, identifying osteoarthritis in its preradiological stages, examining responses of the cartilaginous tissues to treatment and evaluating long-term prognosis. Markers should prove most useful in prospective studies aimed at identifying early changes in cartilage metabolism in humans who are at high risk of developing posttraumatic osteoarthritis.