The oxidation of a methyl or methylene group activated by an adjacent double bond, carbonyl, aromatic, etc. into a carbonyl or alkoxyl group using selenium dioxide as the mild oxidant is generally known as the Riley oxidation. All the mechanistic features of this reaction have been discussed. This reaction has been carried out at different temperatures and it has been reported that recovered selenium can be oxidized and used again. This reaction has wide application in organic synthesis, especially for the preparation of allyl alcohol.