The synovial membrane acts on the cartilage at the levels of the synovial cells, the production of soluble factors and the cartilage components. The synovial membrane is at the same time a trophic, an immunologic and a defense organ. One does not oppose anymore the aspects found in normal synovium to those found in osteoarthritis and in rheumatoid arthritis. Synovial reactivity against the cartilage components is mediated by specific initiating mechanisms leading to an amplification process. Various soluble factors regulate this cooperation at the articular and the systemic levels. The dysregulation between these factors plays a role in the pathogenesis of joint diseases.