In comparison to the solid-vacuum interface, i.e. the clean, well-defined surface of a solid, other solid interfaces are of much more practical importance. The solid-liquid interface, for example, plays a major role in electrochemistry and biophysics. Studies of that particular interface have a long tradition in physical chemistry. A detailed treatment of solid-liquid interfaces is far beyond the scope of this text although certain general concepts, e.g. that of space-charge layers, are similar to those of the solid-vacuum and solid-solid interface.