A peroxide is a compound containing an oxygen–oxygen single bond or the peroxide anion, O2−
2. The O−O group is called the peroxide group or peroxo group. In contrast to oxide ions, the oxygen atoms in the peroxide ion have an oxidation state of −1.

The simplest stable peroxide is hydrogen peroxide. Superoxides, dioxygenyls, ozones and ozonides are considered separately. Peroxide compounds can be roughly classified into organic and inorganic. Whereas the inorganic peroxides have an ionic, salt-like character, the organic peroxides are dominated by the covalent bonds. The oxygen–oxygen chemical bond of peroxide is unstable and easily split into reactive radicals via homolytic cleavage. For this reason, peroxides are found in nature only in small quantities, in water, atmosphere, plants, and animals. Peroxide ion formation has recently been highlighted as one of the main mechanisms by which oxides accommodate excess oxygen in ionic crystals and may have a large impact on a range of industrial applications including solid oxide fuel cells.