Cytokine expression in the inflamed synovial membrane of patients with rheumatoid arthritis and other forms of chronic inflammatory arthritis and other forms of chronic inflammatory arthritis leads to formation of osteoclasts. These cells are primarily involved in the resorption of mineralized cartilage and bone and thus participate in joint damage in the course of chronic arthritides. Osteoclastogenesis in the synovial membrane is driven by cytokines such as RANKL, MCSF but also classical proinflammatory mediators such as TNF, IL-1 and IL-6. Inhibition of osteoclast formation has proven as an effective approach to inhibit structural damage in experimental arthritis and preliminary data suggest that such approaches are also effective in human RA.