This chapter reviews some of the basic concepts and latest results concerning enzymes in reverse micelles. Three methods have been proposed and utilized for the solubilization of enzymes in reverse micellar solutions. The “injection method’ is the simplest: the hydrocarbon solution of the surfactant is prepared and to this solution a small amount of a concentrated aqueous protein solution is added. The second method is to solubilize proteins in a hydrocarbon micellar solution and is called the “phase-transfer method.” The third method is transfer from the solid state of proteins, which is particularly suited for water-insoluble proteins. The phenomenon of “superactivity” of certain hydrolases, such as a-chymotrypsin, ribonuclease, and lysozyme, is present only at very small water content. Under this condition, the conformational perturbations are the largest and the micelle should be the smallest—i.e., the least hospitable.