This review concerns selective oxidation catalysts which often exhibit an increase in activity or/and selectivity as the reaction proceeds to reach its steady state. The objective is to discuss the possible reasons of this phenomenon. The evidence is presented that the activation of most of the reviewed catalysts was caused by the reaction-induced restructuring observed to occur at the surface as well as in the bulk of catalyst. Different mechanisms, such as removal of weakly bound oxygen, creation of active sites, deposition of carbonaceous material, formation of new phases or overlayers, remote controlled cooperation between separate phases and others, are discussed. These changes in catalyst composition are to be taken into account both for modelling the reaction mechanism and kinetics.