The aim of this paper is to discuss the theoretical expressions for permeation transients in terms of the insertion reaction mechanism (one-step or two-step mechanism) and kinetics. The concentration and flux step boundary conditions are first discussed for potentiostatic charging. The intermediate case of decreasing input flux is also dealt with. On the other hand, constant flux or increasing flux can be envisaged under galvanostatic charging conditions. In contrast, the constant concentration boundary condition implicitly used in many galvanostatic permeation studies is incorrect if hydrogen saturation is disregarded. The diffusion coefficient of hydrogen in palladium, determined by different authors from permeation studies and electrochemical impedance spectroscopy (EIS) data, is finally discussed based on the insertion reaction mechanism and kinetics.