The best known NADPH oxidase is that of phagocytes‐ neutrophils and monocytes. In these cells, the enzyme manufactures large quantities of O2 ‐ and other reactive oxidants that are used for the purpose of killing invading microorganisms. Recent studies, however, have suggested that a number of other tissues contain NADPH oxidases. In contrast to the very vigorous production of oxidants by phagocytes, rates of oxidant production by these other cell types are quite low. Oxidant production by these cells is generally thought to serve a signaling function.