In this article, the key issue of the catalysts for hydrogenation of aromatic and aliphatic carboxylic acids to the corresponding aldehydes was reviewed. For the hydrogenation of aromatic carboxylic acids, metal oxides, such as ZrO2, CeO2, ZnO and MnO, show high activity and selectivity to aldehydes. On the other hand, for aliphatic carboxylic acids which have two alpha -hydrogen atoms, these metal oxides show low selectivity for hydrogenation, because undesirable ketonization occurs. For the hydrogenation of aliphatic carboxylic acids, Cr2O3 and the partially reduced Fe2O3 show high selectivity. Especially, the highly pure Cr2O3 exhibits superior chemo-selectivity to aldehyde without C=C bond migration or ketonization. The chromium-modi fled ZrO2 catalyst for manufacturing aromatic aldehydes and the highly pure Cr2O3 catalyst for manufacturing aliphatic aldehydes have been commercially used in industrial processes. A new aspect of direct hydrogenation catalysts is briefly reviewed, and the reaction mechanism for hydrogenation of carboxylic acids is also discussed.