Hydrogen absorption reaction into a metal was presented from the mechanistic and kinetic point of view. It is a complex process, usually accompanied by hydrogen UPD and hydrogen evolution. Influence of the hydrogen formed at positive overpotentials in solution, influence of the quantity of hydrogen absorbed and related diffusion limitations, presence of the hydrogen UPD in parallel with the OPD adsorption and absorption reactions, and the nature of the kinetic limitations were analyzed. Possibilities and limitations of the determination of the mechanism and its kinetic parameters were discussed.