The effect of additives on metal oxide and silver catalysts to improve the activity and selectivity of various processes is considered. Additives influence the kinetic parameters of a process (rate and activation energy) and the reaction mechanism (structure of surface compounds, energy of their bond with the catalyst). Modification causes the formation of new compounds in the bulk and at the surface of the catalyst, thereby altering the composition of active sites and inducing formation of other sites. The effects of species added to multicomponent oxide systems, as well as various aspects of modification by acid-base additives are considered. Specific features of the influence of additives and the role of modified multifunctional catalytic systems are also discussed.