In arthritic diseases, the stability of the extracellular matrix of articular cartilage is compromised by extensive proteolytic breakdown associated with alterations of synthesis of the proteins of the tissue leading to cartilage loss. This article reviews developments in assays of biochemical markers of cartilage matrix turnover and studies investigating their use. Because type II collagen and aggrecan are the most abundant proteins of the cartilage matrix, current biochemical markers are based mainly on immunologic reagents detecting their synthesis and degradation. Clinical studies indicate that some markers of type II collagen may be useful to predict disease progression in osteoarthritis and rheumatoid arthritis. Conversely, major achievements have been made in the development of immunoassays detecting the various fragments of aggrecan released by matrix metalloproteases or aggrecanases, but their use has been limited mostly to investigating cartilage turnover in ex vivo experiments. Because of the complexity of the mechanisms involved in arthritic joint damage, only a combination of different biochemical markers reflecting the various aspects of synthesis and degradation of matrix molecules will likely provide efficient cartilage turnover monitoring.