Biological markers for osteoarthritis (OA) are indicators of articular tissue metabolism, measuring the levels of molecules derived from joint structures into synovial fluids, serum and urine. Radiological findings are the basis of diagnosis of OA, but have a weakpoint visualizing the figures that have already occurred. For assessing disease activity or monitoring disease progression, estimation of articular metabolism using biological markers is very important. Synovial fluid reflects specifically the status of the punctured joint, however invasive. Less-invasive newly developed serum and urine markers appear to be also useful for the evaluation of OA.