Glycyrrhizin (GL), the well-known sweet saponin of licorice, has been used as a food-additive and as a medicine. Its aglycone, glycyrrhetinic acid (GA) showed antiinflamatory, antiulcer and antiviral properties. GA is now produced form GL by acid hydrolysis. However, it is difficult to obtain GA in a good yield by this method, because many by- products are also produced. To produce GA from GL, we have screened microorganisms for activity to hydrolyze GL. Then, the selected strain was subjected to different studies concerning the effect of some physiological and biochemical parameters on its ability to transform GL, in order to achieve the maximal conversion rate. There has been increased interest in the field of microbial transformation and biosynthesis of organic compounds on the strength of new methods, especially immobilization of microbial cells. Thus, the possibility of GL bioconversion by immobilized microbial cells and spores using different immobilization methods was investigated. This book can be considered to be the first report describing the possibility of using the immobilized microbial cells and spores in the bioconversion of GL.