Clays are very versatile materials that can be used as absorbents, ion exchangers, decolorizing agents, catalyst supports and catalysts. Clay-catalysts are interesting materials, not only for the low cost of the raw material, but also for their lack of harmful effects on the environment. The study of modified clays has attracted interest due to the structure and dimension of their pores, which appear to be suitable for the conversion of large molecules. Acid-activation is one of the modifications that can be made to clays. It causes the disaggregation of clay particles, elimination of impurities and dissolution of external layers, altering the chemical composition and structure of clays. The main consequences of acid activation are an increase of surface area, porosity and number of acid sites compared to the parent clays.