In 1899, Bayer and Vlliger showed that the oxidation of alicyclic ketones, menthone, tetrahydrocarvone, and camphor with permono-sulfuric acid led to the formation of lactones. Further studies using a variety of ketones or aldehydes and hydrogen peroxide or peracids in various media have established that the oxidation is of wide applicability. The oxidation, the Baeyer-Villiger reaction, is the subject of this review. As the oxidation normally employs mild conditions, gives reasonable yields, and shows a high degree of selectivity, it has proved useful in a variety of both synthetic and degradative studies. Recent investigations have led to a better definition of favorable experimental conditions and have extended the scope of the reaction.