The deposition of calcium-containing crystals in articular tissues is probably an under-recognized event. Clinical observations indicate that an exaggerated and uniquely distributed cartilage degeneration is associated with these deposits. Measurements of putative markers of cartilage breakdown suggest that the presence of these crystals magnifies the degenerative process. In vitro studies indicate two potential mechanisms by which crystals cause degeneration. These involve the stimulation of mitogenesis in synovial fibroblasts and the secretion of proteases by cells that phagocytose these crystals. Approaches that might ameliorate the degenerative process may ensue from new information about how crystals form and how they exert their biologic effects.