Osteoarthritis (OA) is a complex process affecting many different joint areas in the body. From a pathophysiological point of view some features are crucial for the diagnosis, such as cartilage fibrillation and thinning, subchondral sclerosis and the presence of osteophytes. From a clinical perspective, OA is the most prevalent rheumatic joint disorder, causing pain and stiffness of the joints and, for the individual, impaired function and health status. The aim of this chapter is to present current knowledge of definitions of OA, its presence in different populations and in different joint areas (the back excluded). Furthermore, methods of diagnosing and delineating clinically relevant forms of OA, now and in the future, are presented as well as current knowledge of the risk factors for developing and the factors for preventing OA.