Joint disease is one of the most frequent causes of reaching a state requiring care by others. Thus, it is very important to cope with osteoarthritis (OA) in order to maintain the physical independence of elderly people. Xp findings of OA in the knee, which is the most commonly affected joint, do not correlate well with pain ; hence, on the diagnosis, MRI is useful to know the pathology of subchondral bone, cartilage or meniscus. In OA cartilage, increased degradation and decreased production of major matrix components, like type II collagen and aggrecan occur, as well as chondrocyte hypertrophy or apoptosis. In the meantime, general changes in the aging process, such as the changes of autophagy or energy metabolism, have also recently been reported. Since OA treatments at present are just against symptoms, like by NSAIDS or arthroplasty, the development of new treatment methods is expected, based on the clarification of the pathology, including consideration of the relations with aging of bone and muscles, or with metabolic syndrome.