text: Kinetic isotope effect (KIE) is a measurement of the reaction rate of isotope-labeled reactants against the more common natural substrate. Kinetic isotope effect values are a ratio of the turnover number and include all steps of the reaction. Intrinsic kinetic isotope  values stem from the difference in the bond vibrational environment of an atom in the reactants at ground state to the environment of the atom's transition state. Through the kinetic isotope effect much insight can be gained as to what the transition state looks like of an enzyme-catalyzed reaction and guide the development of transition state analogs.