Osteoarthritis (OA) is one of the most common chronic diseases and the most frequent cause of rheumatic complaints in the older population. While still regarded primarily as an expression of a degenerative disease of cartilage accompanying the aging process, newer concepts in many areas have emerged. In particular, these concepts relate to cellular interactions among joint components, biochemistry and ultrastructure of cartilage, biomechanics, pharmacology and antiarthritic drugs, surgical approaches to the repair of arthritic joints, and population studies. However, it is still very difficult at this time to put together a coherent picture of the evolution of OA, and this profoundly limits our understanding of the disease process. In population studies we often rely on physical inspection of joints or complaints of pain, both of which may be of limited value despite extensive damage to the articular cartilage. This makes us dependent on x-rays for case finding, and these are costly and difficult to do. Longitudinal studies are not feasible because we are talking about a survey that should extend for several decades. We have limited access to samples of joint components for biochemical studies in the 20 to 40-year-old population, although surgical intervention for menisceal injury should offer an opportunity to observe conditions where the evolution of OA is often accelerated. This discussion, written by an internist, is an attempt to survey aspects of the present state of our knowledge, albeit incomplete, of OA involving the joints of the extremities. It is hoped that orthopaedic surgeons in particular will be stimulated to explore further the evolution of OA, an area to which they have contributed so significantly.