Publisher Summary This chapter discusses biological applications of two classes of clays, cationic and anionic ones. The chapter describes their structure, physical and chemical properties, and preparation. Clay minerals have attracted a great deal of attention from a very wide range of scientific and industrial fields because both of natural abundance and of exhaustless and unlimited potentials. A variety of clay minerals is found in nature. In general, clay minerals could be roughly divided into three classes by ion exchange property. These are nonionic, cationic, and anionic clays. Non-ionic clays with no true ion exchange capacity include kaolinite, serpentine, chlorite, illite, pyrophyllite, and talc. Cationic clays with cation exchange capacity comprise many alumino-silicate clays such as vermiculites, smectites, and swelling micas. Biological applications of cationic clays are found in many fields such as pharmaceutics, medicine, cosmetics, spas, food, fodder, and pesticide into which cationic clays are directly involved as active principles or excipients.