Publisher Summary Quinones are diketones that are derived from dihydro aromatic compounds, in which the two carbonyl groups are connected by a system of conjugated double bonds. The two ketonic groups in polycyclic compounds might be in the same ring or in different rings. Easy reducibility to aromatic dihydroxy compounds is characteristic of o- and p-quinone and intermediate compounds or pseudo-quinols are known. The diphenols are readily oxidized to quinones. In solution, the semi-quinone half-way compound might be studied as a free radical, that is, an odd-electron, paramagnetic compound with characteristic spectroscopic properties. Quinhydrones are double compounds in which one molecule of quinol and one of quinone are united to form a diamagnetic complex. The oxidation-reduction potentials of quinones are important in many biochemical systems.