Aggrecan is essential for the normal function of articular cartilage and intervertebral disc, where it provides the ability for the tissues to withstand compressive loading. This property depends on both the high charge density endowed by its numerous chondroitin sulfate and keratan sulfate chains and its ability to form large molecular aggregates via interaction with hyaluronan. Degradation of aggrecan via the action of proteases takes place throughout life and the degradation products accumulate in the tissue and impair its function. Such degradation is exacerbated in degenerative or inflammatory joint disorders. The use of antibodies recognizing the various regions of aggrecan and the neoepitopes generated upon proteolytic cleavage has shown that matrix metalloproteinases and aggrecanases, members of the ADAMTS family, are responsible for aggrecan degradation, both throughout life and in disease. By using immunoblotting techniques, it is possible to determine the extent of aggrecan degradation and to identify the degradation products that have accumulated in the tissue, and immunohistochemistry allows the location of the aggrecan degradation to be established.