This review is concerned mainly with the three redox-related, but chemically distinct, species NO-, NO. and NO+, with greatest emphasis being placed on the chemistry and biology of the nitroxyl ion. Biochemical routes for the formation of nitroxyl ion and methods for showing the intermediacy of this species are discussed, together with chemical methods for generating nitroxyl ion in solution. Reactions of nitroxyl ion with NO., thiols, iron centres in haem and with dioxygen are reviewed The significance of the reaction between NO- and dioxygen as a source of peroxynitrite is assessed, and attention drawn to the possible significance of the spin state of the nitroxyl ion in this context. The biological significance of nitrosation and the importance of S-nitrosothiols and certain metal nitrosyl complexes as carriers of NO+ at physiological pH is stressed. Some features in the chemistry of peroxynitrite are noted.