Inflammatory arthritis has the potential to cause irreversible erosive damage to cartilage and bone. This may occur very early in the course of the disease. At present it is not possible at diagnosis to identify those patients who will develop erosive damage. If this were possible, it would enable aggressive therapy to be targeted to those patients at greatest risk. There is therefore a need for sensitive markers to detect and quantify joint damage at as early a stage as possible. In this chapter we review potential biochemical markers of such damage and assess their clinical usefulness.