Hydrogen-atom transfer (HAT) reactions involve the transfer of a hydrogen atom from one species to another, and they play a crucial role in many chemical and biological processes, including organic reactions and catalysis, drug development, and sustainable energy solutions. Theoretical predictions of the reaction rates of HAT reactions are challenging. A popular approach is transition state theory (TST). Keys to the success of TST are variational determination of the transition state, accurate estimation of the free energy difference between the reactants and the transition state, and a reliable tunneling transmission coefficient. TST, in conventional or variational form, has been employed for qualitative, semi-quantitative, and sometimes quantitative predictions of many chemical and biological reactions in both the gas phase and the condensed phase. However, the treatment of quantum mechanical zero-point energy (ZPE) and tunneling can make the attainment of accurate results more arduous for HAT reactions than for reactions that do not have large hydrogenic motion in the reaction coordinate.