A new concept has been proposed for understanding the catalytic activity for the hydrogen evolution reaction on transition metals and alloys, based on a series of experiments on the hydrogen overpotential and its interpretation using the electronic structures calculated by the DV-Xa cluster method. In this concept, the transferred direction of charges between a base metal and an alloying element is substantial since this is responsible mainly for the hydrogen overpotential change with alloying If the alloying element is more electronegative than the base metal, the charge transfer occurs from the base metal to the alloying ekment. Excess electrons are then located near the alloying element. In such a case the alloying ckmcnt provides preferable sites for hydrogen evolution reactions, since the excess electrons promote proton discharge. As the result, the hydrogen overpotential of the alloy reveals the value characteristic of the alloying element rather than the base metal. On the other hand, if the alloying ekment is less ckctroncgative than the base metal, charge transfer takes place in reverse, namely from the alloying element to the base metal. In this case, the base metal behaves as the active site for hydrogen evolution, and hence the hydrogen overpotential of the alloy is close to that of the base metal and less dependent on the alloying element. The Fermi energy level is also shown to be correlated with the hydrogen overpotcntial of alloys when the alloying element is more electronegative than the base metal. The observed charactctistics of hydrogen overpotential for Ti, Fe and Ni alloys can bc understood consistently following this concept.