In the presence of suitable catalysts esters react with hydrogen to form alcohols. The reaction is reversible, the relative concentrations of esters and alcohols at equilibrium being determined by the temperature and particularly by the pressure of hydrogen. The conversion of esters to alcohols may proceed either by hydrogenation or by hydrogenolysis. There is no experimental basis for the choice of one hypothesis over another, the use of the term hydrogenation in this chapter is matter of convenience and does not imply a choice of mechanism. The catalyst most suitable for the preparation of alcohols from esters by reacting with hydrogen under pressure is copper chromium oxide. This chapter deals almost exclusively with this catalyst, However, certain other catalysts of limited usefulness are considered.