This article introduces the basic considerations of osteoarthritis. It contains three parts: 1. the introduction 2. the epidemiologic study 3. the pathologic change. Osteoarthritis is a subset of the rheumatic disorders. It is not an inevitable event of aging, but is likely the result of prolonged exposure to pathophysiologic processes that occur much earlier in life. Although the majority of OA patients in younger age groups may have mild symptoms, OA will result in severe disability in older populations. Recent advances in treating OA have made it much effectively in relieving symptoms and restoring the living or working abilities of patients. The prevalence rate of OA in any population over 35 y/o is about 60%. The rate was found significantly increased with advancing age. The disease occurs much frequently in women over 45 y/o. Sex also influences the pattern of distribution of OA. Mechanical factors, systemic factors, ethnic groups, heredity and inflammation were discussed regarding their relationship with OA. Cartilage degeneration is a common feature in preclinical stage of OA. Fibrillation, horizotal splitting and cartilage thinning from grinding damage in intermediate stage will further expose the calcified tissure and lead to late stage. In late stage OA, osteosclerosis of the subarticular bone with some interspersed cysts were striking pictures. Plugs of new fibrous and chondroid tissures interspersed over the exposured bone will play an important role in remodeling and repairing activities.