A new theoretical method for the analysis of the superexchange coupling and localization of electron tunneling pathways in long distance electron transfer reactions is introduced. The new method allows one to examine spatial distribution of microscopic quantum mechanical tunneling currents flowing through individual atoms, or to evaluate the relative probability that the tunneling electron will pass through an individual atom, in the intervening medium between donor and acceptor in the course of an electron transfer reaction. It is shown how the interatomic tunneling currents introduced in this paper can be calculated using methods of quantum chemistry. The method provides a rigorous theoretical framework for the description of the tunneling process in long-range electron transfer reactions in proteins. The relation of the present theory of tunneling currents to the theory of pathways of Beratan and Onuchic is discussed.