Phosphorus is a major biogeochemical element controlling growth in many ecosystems. It has presumably been an important element since the onset of life. In most chemical and biochemical considerations, phosphorus is synonymous with phosphates, a pentavalent oxidation state that includes the phosphate backbone of DNA and RNA, as well as major metabolites such as ATP. However, redox processing of phosphates to phosphites and phosphonates, and to even lower oxidation states provides a work-around to many of the problems of prebiotic chemistry, including phosphorus’s low solubility and poor reactivity. In addition, modern phosphorus cycling has increasingly identified reduced P compounds as playing a role, sometimes significant, in biogeochemical processes. This suggests that phosphorus is not redox-insensitive and reduced P compounds should be considered as part of the phosphorus biogeochemical cycle.