Zeolites are included in the group of molecular sieves, because they preferentially adsorb inside the pore structure molecules with a smaller diameter than their pore size, while excluding larger molecules. There are more than 220 known zeolite framework types, approved by the Structure Commission of the International Zeolite Association. Many zeolites occur as natural minerals, but some of the synthetic ones, in their acidic form, are probably the most important heterogeneous catalysts used by the chemical industry. One of the main properties of zeolites (and other molecular sieves) is the shape selectivity, arising from the confined environment in which the molecules are converted. The well-defined, open crystalline structure of zeolites, with a regular pore distribution, has properties that are appreciated in industrial catalysis since the early 1960s. In ion exchange applications, zeolites are used for water softening, radionuclide separations and wastewater treatments. Many industrial processes use zeolite catalysts to increase the value of different petroleum cuts.