This paper deals with a review of the literature on the catalysis by acid-treated clays. The literature which has appeared in the past 8 years has been given prominence. A variety of reactions catalyzed by acid-treated clays have been covered, with emphasis on industrial applications. The reasons for successful exploitation of acid-treated clays as catalysts have been brought out. A comparison of the performance of the acid-treated clays with some of the other heterogeneous acid catalysts, e.g. ion-exchange resins, has been done wherever applicable.