Osteoarthritis (OA) is considered to be linked to obesity and body fat mass. Recent investigations, however, are aimed at clarifying the roles of adipose tissue-derived proteins and a wide variety of lipid mediators, including fatty acids, sphingolipids, and eicosanoids, in cartilage degradation in OA, in addition to the effects body weight itself. Here, we review recent progress in studies of OA, focusing on the potential role of lipid mediators in articular cartilage and introducing the concept that "OA is a metabolic disease" in which lipids essentially contribute to the pathophysiology of cartilage degradation.