Some of the factors that may affect the chemoselectivity in the hydrogenation of unsaturated carbonyl compounds with heterogeneous catalysts are presented and discussed. Among them, the presence of a second metal, either added in an ionic form in the reaction medium or alloyed in a bimetallic catalyst is discussed in the first part of the paper. The role played by ligands and their effects upon the electronic structure of the metal particles and hence upon their selectivity is discussed in a second part. Finally the geometric and steric effects favouring the hydrogenation of the carbonyl group of aldehydes are presented.