This book describes a new phenomenon of global importance - magnetic isotope effect. In contrast to the well-known classical isotope effect which originates from the difference in chemical reactivity of reagents with light and heavy nuclei, magnetic isotope effect results from the difference in reactivity of reagents with magnetic and nonmagnetic isotopic nuclei. It gives a comprehensive description of the new isotope effect and its exhibition in chemical and biochemical reactions as well as future prospects and potential of the use in biomedicine (in particular, for the stimulation of ATP productions in living organisms by using stable magnetic isotope magnesium-25).