Rice-Ramsperger-Kassel-Marcus (or simply RRKM) is a chemical kinetics theory allowing the estimation of unimolecular reaction rates from a few parameters of the potential energy surface. RRKM theory is an improved form of the Rice-Ramsperger-Kassel (RRK) theory in which the rate an excited reactant molecule breaks down is treated as a function of the energy it contains. The RRKM theory was developed from the RRK theory by taking Eyring’s transition state theory, and the way the various normal-mode vibrations and rotations contribute to reaction, into account. The RRKM theory assumes that the rate is proportional to the number of ways of distributing the energy among the internal degrees of freedom of the reactant, in such a manner that the critical energy is localized in one particular degree of freedom. It can be a powerful tool for studying gas-phase molecular reactions, such as those that occur in planetary atmospheres or in the interstellar medium.