Biologic carbon–sulfur chemistry is central in carbon chemical biology. Key metabolites include thiols, thioethers, alkyl sulfonium cations, disulfides, persulfides, and acyl thioesters. The ability of organisms to work with different oxidation states of sulfur bring sulfuryl anhydrides, sulfate esters, and sulfenamides into metabolic pathways. The oxidation of thiols occurs in two modes. One is to oxygen-containing sulfenates, sulfinates, and sulfonates. The second involves persulfides and disulfides. The three most common carbon–sulfur functional groups in biology are the thiol of cysteine, the thioether in methionine, and the trigonalized sulfonium cation in S-adenosylmethionine.