Phosphorus, one of the six chemical elements required for all life forms, is present largely as inorganic phosphate salts, in surface rock and as dissolved sulfate dianions, reaching 28 mM concentrations in the global oceans. Essentially all the chemical biology of phosphorus, as inorganic phosphate and simple derivatives, phosphoric anhydrides, phosphate monoesters and phosphate diesters, occurs in the P^V oxidation state with essentially no redox change at the central phosphorus atom in the various biological phosphate forms.