In recent years molecular sieve catalysts have assumed an increasingly important role in industrial catalysis, Applications of zeolite catalysts are expanding from the traditional petroleum refining to new and improved fuel processing applications, and to new roles in both the petrochemical and chemical industries. Up to the present, all commercial applications of zeolite catalysts have been carried out with aciaic zeolites. Recent investigations of zeolite chemistry revealed several important features which appear common to both alkali and acidic zeolites. This new chemical evidence raises the possibility that the underlying physicochemical features of both types of zeolites play a role in catalysis.