Molecular transformation (chemical reaction) as an electron-nuclear rearrangement of the reactants into the products is the heart of chemistry, the central event which all chemistry circulates about. It is selective to the nuclei, both in mass and spin. Nuclear mass selectivity of reactions results in classical isotope effect (CIE), the remarkable phenomenon which continues to play a unique role and has served served for many years as a powerful and reliable tool of mechanistic chemistry and biochemistry. Another breakthrough of similar scale and importance is the discovery of nuclear spin selectivity of chemical reactions, which is the dependence of the reaction rates on the nuclear spin and nuclear magnetic moment of the reactants. In contrast to CIE, which is governed by chemical energy of the starting and transition states of reactant molecules, this new isotope effect is controlled by magnetic interactions, so it was christened the magnetic isotope effect (MIE). The general principles of tuning of the reactions to MIE are discussed, particularly by microwaves, tunable on frequency and amplitude, to selectively modify and control chemical reactivity.