The author focuses on the biological significance of B-oxidation of fatty acids. Early studies revealed that there is a subsidiary pathway for O-oxidation of fatty acids when O-oxidation is blocked. Many studies demonstrated that the B-oxidation serves to provide succinyl-CoA for the citric acid cycle and for gluconeogenesis under conditions of starvation and diabetes. Acylglucosylceramides which are composed of linoleic acid, long chain B-hydroxy fatty acids, eicosasphingenine (or trihydroxyeicosasphingenine) and glucose, are responsible for normal epidermal permeability function in the skin. It is observed that B-and (B-1)-oxidation of fatty acids are related to energy metabolism in some laboratory animals such as musk shrews and Mongolian gerbils. Studies confirmed that B-and (B-1)-oxidation of fatty acids play crucial roles in the production of insect pheromones of honeybees and in the formation of biopolyesters of higher plants. In addition, the biological significance of B-oxidation of prostaglandins and leukotrienes is described.