过渡状态理论 （Transition state theory，TST）解释基元化学反应的反应速率。该理论假设在反应物和活化的过渡态络合物之间有一种特殊的化学平衡（准平衡）。

TST最初用于定性地解释化学反应是怎幺发生的。TST就其计算绝对反应速率常数的本来目的来说不算十分成功。因为，绝对反应速率常数的计算需要精确的知道势能面。 但是，如果一个特定的反应的速率常数可以用经验的方法测定的话，该理论能够成功地计算了标准活化焓（ΔH），标准活化熵（ΔS），和标准活化吉布斯自由能（ΔG）。（所表示的是在过渡状态时所感兴趣的值。）

Transition state theory (TST) explains the reaction rates of elementary chemical reactions. The theory assumes a special type of chemical equilibrium (quasi-equilibrium) between reactants and activated transition state complexes.

TST is used primarily to understand qualitatively how chemical reactions take place. TST has been less successful in its original goal of calculating absolute reaction rate constants because the calculation of absolute reaction rates requires precise knowledge of potential energy surfaces, but it has been successful in calculating the standard enthalpy of activation (ΔH), the standard entropy of activation (ΔS), and the standard Gibbs energy of activation (ΔG) for a particular reaction if its rate constant has been experimentally determined. (The notation refers to the value of interest at the transition state.)