The porous semiconductor electrode provides an interesting example for the theme of this volume: electrochemistry at the nanoscale. In porous etching, the anodic reaction can be considered to occur at an array of “nanoelectrodes”, the pore tips, while the remainder of the porous matrix remains electrochemically inactive. In this case, conditions are clearly different from those at a macroscopic surface. Porous electrodes can also exhibit another aspect, one in which charge transfer is not restricted to the pore fronts; instead, the whole internal surface of the matrix acts as an electrode with a very large area but with a reduced “thickness”, corresponding to the dimensions of the pore wall. Such small dimensions, which can even lead to size quantization, play a critical role in the electrochemistry. In this chapter, we consider the factors that decide whether the electrochemical reaction occurs exclusively at the pore fronts or at the whole internal surface of the porous layer. We review the electrochemistry of the two cases and related chemical and physical properties. In addition, we compare some results of porous-etched single crystals with those of nanoporous electrodes made by deposition from colloidal suspension.