Electron tunneling is well recognized in physics. It was first proposed by Nordheim (1) in 1927 and was quickly applied to electron movements by many early theorists. For example, Nordheim (1) applied it to thermal emission of electrons. Oppenheimer (2) applied it to the problem of field emission of electrons in 192R. Nordheim (3), Frenkel & Joffe (4), and Wilson (5) applied it to rectifying barriers in 1932. Zener (6) used it to explain avalanche breakdown in semiconductor junctions in 1934. Giaever (7) first observed tunneling currents from superconducting metals separated through oxide barriers. Josephson (8) predicted, and it has been confirmed ex perimentally, that electrons tunneling through junctions in superconducting rings ex hibit ccrtain quantum-mechanical oscillatory phenomena. Mead and co-workers (9) have studied electron tunneling through barriers nearly 100 A thick and confirmed the quantum-mechanical relationships. Practical appliances have resulted such as tunnel diodes [Esaki (10) 1958], and numerous devices (11) for accurately measuring small voltages or magnetic fields, or detecting infrared radiation that use the Josephson effect. Still being developed are computer memories (12) that use the Josephson effect, and a method of spec troscopy of minute amounts of organic or biological materials embedded in a tunnel barrier (13). In chemistry, the tunneling of electrons is also well recognized. Gurney (14) proposed in 1931 that electrolysis involved electron tunneling through the electrode surface and showed how the necessity to match energy levels could produce the phenomenon of overvoltage. His theory was extended by Horiuti & Polanyi (15) in 1935. In 1939 Mott (16) suggested that the oxidation of aluminum is controlled by electrons tunneling through the oxide layer from the metal to adsorbed O2, In 1940 Libby (17a) suggested electron tunneling to explain the rapid exchange between MnO';and MnO�in solution. In 1952 (17b), Libby used tunneling, along with a requirement of solvent rearrangement, to satisfy the Franck-Condon principle and explain such facts as that ferriand ferrocyanide ions exchange electrons much more rapidly than ferric and ferrous ions. R. A. Marcus (18), in a series of papers,