Zeolites have been used as catalysts in industry since the early 1960s. The great majority of commercial applications employ one of three zeolite types: zeolite Y (faujasite); mordenite; ZSM-5. By far the largest use of zeolites is in catalytic cracking, and to a lesser extent in hydrocracking. Table 1 presents some data showing the commercial importance of this field [1]. The data are for United States refineries only and must be multiplied by a factor to arrive at worldwide use. Better than 90% of free-world cracking units now use zeolite catalysts. For many years it had been assumed that crystalline aluminosilicates with their uniform pore structure would make inferior catalysts to amorphous silica-slumina with a rather wide pore size distribution. The tremendous acid activity of hydrogen zeolites also was not recognized. Rabo and co-workers [2] showed at the 2nd International Congress on Catalysis that hydrogen exchanged faujasites possessed good isomerization ability, but commercial application in catalytic cracking became feasible only after Plank and Rosinsky at Socony-Mobil Oil Co. succeeded in stabilizing zeolite Y against steam and heat sintering by exchange with rare earth ions and by separating zeolite crystallites by incorporating them into a silica-alumina matrix which provided a heat reservoir along with some synergistic cracking effects. Modern cracking catalysts comprise 10–40% rare earth exchanged H-Y zeolite dispersed in a matrix of silica-alumina, semisynthetic clay, or natural clay.