Bicarbonate, or hydrogen carbonate, is a simple single carbon molecule that plays surprisingly important roles in diverse biological processes. Among these are photosynthesis, the Krebs cycle, whole-body and cellular pH regulation, and volume regulation. Since bicarbonate is charged it is not permeable to lipid bilayers. Mammalian membranes thus contain bicarbonate transport proteins to facilitate the specific transmembrane movement of HCO3(-). Bicarbonate ion is an anion that consists of one central carbon atom surrounded by three oxygen atoms in a trigonal planar arrangement, with a hydrogen atom attached to one of the oxygens. The bicarbonate ion carries a negative one formal charge and is the conjugate base of carbonic acid, H2CO3. The carbonate radical is an elusive and strong one-electron oxidant. Bicarbonate in equilibrium with carbon dioxide constitutes the main physiological buffer. The bicarbonate-carbon dioxide pair stimulates the oxidation, peroxidation and nitration of several biological targets. The demonstration that the carbonate radical existed as an independent species in aqueous solutions at physiological pH and temperature renewed the interest in the pathophysiological roles of this radical and related species. The carbonate radical has been proposed to be a key mediator of the oxidative damage resulting from peroxynitrite production, xanthine oxidase turnover and superoxide dismutase1 peroxidase activity. The carbonate radical has also been proposed to be responsible for the stimulatory effects of the bicarbonate-carbon dioxide pair on oxidations mediated by hydrogen peroxide/transition metal ions. The ultimate precursor of the carbonate radical anion being bicarbonate, carbon dioxide, peroxymonocarbonate or complexes of transition metal ions with bicarbonate-derived species remains a matter of debate. The carbonate radical mediates some of the pathogenic effects of peroxynitrite. The carbonate radical as the oxidant produced from superoxide dismutase (EC 1.15.1.1, SOD1) peroxidase activity. Peroxymonocarbonate is a biological oxidant, whose existence is in equilibrium with hydrogen peroxide and bicarbonate (PMID: 17505962, 17215880). Hydrogen carbonate is found to be associated with hawkinsinuria, which is an inborn error of metabolism.