Henry Eyring was a Mexico-born United States theoretical chemist whose primary contribution was in the study of chemical reaction rates and intermediates. Eyring developed the Absolute Rate Theory or Transition state theory of chemical reactions, connecting the fields of chemistry and physics through atomic theory, quantum theory, and statistical mechanics.

In chemistry, 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.