Reactions where hydrogen atoms are transferred to an unsaturated substrate (X = Y) to yield HX-YH and their microscopic reverse have a very wide scope, depending of the H atom source (H2 or a HX′-Y′H donor) and the nature of X = Y (non-polar, e.g., alkene, alkyne, diene, etc.; or polar, e.g., aldehyde, ketone, imine, nitrile, etc.). The involvement of these processes in complex reaction schemes such as “borrowing hydrogen” transformations provides a powerful tool to the synthetic organic chemists. Various catalysts are active and several mechanisms are available, differing in the H atom donor activation mode and in the sequence of events leading to the H atom transfer to the substrate. A new first-level classification of all mechanisms is proposed here on the basis of the catalyst formal oxidation state change and on how Mn activates the H atom source (oxidative, yielding Mn + 2/2H−; redox-neutral, yielding Mn/H−/H+; reductive, yielding Mn−2/2H+). All known hydrogenation/dehydrogenation/transfer hydrogenation mechanisms are listed on the basis of this new classification method, with illustrative examples.