In Number 20 of Modern Aspects of Electrochemistry, we present chapters whose organization is typical for the series: They start with the most fundamental aspects and then work to the more complex. Thus, Jerry Goodisman gives us an interesting contribution on a subject in which he is one of the pioneers, the electron overlap contribution to the double layer potential difference. Closely related to this theme, but not always imbued with knowledge ofit, is the electron transfertheory, treated in this volume by the experienced author A. M. Kuznetsov ofthe Frumkin Institute. H. P. Agarwal is a well-known figure in the field of faradaic rectification, which he originated, and he now teils us about the more recent thinking in the field. On the other hand, Hector D. Abruna comes relatively new to us, and his field, that of X-ray interactions with electrodes, is new, too, but probably augers the trend for the future. The photoelectrochemical reduction of CO2 , described here by Isao Taniguchi from Kumamoto University, is a subject which will have much practical importance as the greenhouse effect continues. Finally, alu mi nu m in aqueous solutions and the physics of its anodic oxide is a subject which seems ever with us, and is described in its latest guise by Aleksandar Despie and Vitaly P. Parkhutik.