  The unequivocal observation in the early 1980s of a new class of unnocupied
surface states that are bound to the vacuum level of a variety of metal
surfaces has led over the last two decades to an active area of research in
condensed-matter and surface physics. We discuss the key ingredients of the
theory of these so-called image states, which have their origin in the long
range nature of the image potential outside a solid.
