This article describes theoretical treatments and experimental data focused on the rates of interfacial electron-transfer processes at semiconductor/liquid contacts. These systems are of practical interest because such electron transfers are a critical factor in understanding the behavior of photoelectrochemical cells as energy conversion devices. These processes are of theoretical interest because the description of how a delocalized charge carrier in a semiconducting solid reacts with a localized redox acceptor that is dissolved in the liquid electrolyte is a relatively undeveloped area of electron-transfer theory. The general principles of these processes, a discussion of past and present experimental data, and a comparison between theoretical expectations and experimental observations on a variety of semiconducting electrode systems are the main focus of this article.