The enzymes that convert arachidonic acid to prostaglandin H2 are named cyclooxygenase-1 (COX-1) and COX-2. The properties of COX-1 are different from those of COX-2. It was originally thought that the function of COX-1 was involved in physiological phenomena, whereas that of COX-2 was involved in various pathologies. However, studies with COX-2 knockout mouse suggest that COX-2 also plays important roles in development and homeostasis. This chapter focuses on the distinct functions of COX-1 and COX-2.