In this book the modification and characterization of clays has been described. Smectite rich montmorillonite clay samples are selected for the modification and their catalytic activity studies. Clay on suitable modification generate acid sites, which makes clay as a potential heterogeneous acid catalyst. Clay catalysts have been prepared by modifying natural clays into different forms by cation exchange, acid activation and pillaring methods. The clay material was analyzed for its elemental analysis by X-ray fluorescence and the cation exchange capacity by barium sulphate method. The characterization technique is also explained such as X-ray diffraction, BET surface area, thermogravimetric analysis. The acidity of catalysts was studied by n-butylamine back titration and FT-IR spectroscopy techniques in this book for the prepared clay catalysts and the moisture absorption applications are also explained for the prepared clay/Polymer samples.