Zeolites and molecular sieves have been extensively studied and applied in different fields. Their specific crystalline structure and composition make it possible to obtain high-activity stable catalyst systems for a wide range of chemical reactions. There are many investigations on the structural characteristics and adsorption and catalytic properties of zeolites and molecular sieves as well as on the relation between the different properties. The nature of active sites in zeolite catalysts and the mechanism of catalytic processes on their surface belong to the most important problems of the investigations.