Cartilage is not simple, inert, homogeneous, bearing surface but rather a complex, living tissue. It has the capacity to provide a low friction surface, participate in joint lubrication, and distribute loads to subchondral bone, all with a very limited capacity for repair and without evidence of abrasive wear over a lifetime of use. The macromolecules of the cartilage matrix, collagen, proteoglycans, noncollagenous proteins and glycoproteins give it its mechanical and biologic properties, and the cells produce and maintain the matrix. The interaction between the cells and matrix is critical for normal function of the tissue, as are the influences of mechanical loads and motion on the tissue, but study of these important areas is only beginning. Significant advances in the understanding and treatment of joint disease depends on further knowledge of cartilage structure and function as well as the factors that maintain and restore the cartilage matrix.