Publisher Summary When trying to explain activity and selectivity of zeolites, all the reaction steps, i.e. diffusion, adsorption and surface reaction have to be considered. It is apparent that the final behavior of a given zeolite will depend strongly on the diffusion characteristics of the reactants into the pores which, besides shape-selectivity effects, will be responsible for the degree of penetration of reactants through the zeolite crystallites. This, in turn, determines the number of active sites which will not necessarily be discerned by techniques providing information only on the bulk zeolite. Furthermore, the presence of composition gradients throughout the crystal is very important for defining the activity and selectivity of zeolites. Changes in the chemical composition of the zeolite will sometimes produce dramatic changes in the total selectivity as well as in the adsorption selectivity.