The theory of surface states underlies our knowledge of the behavior of electrons at solid surfaces, which is of paramount importance in understanding the mechanisms by which solid-state devices operate and catalytic reactions proceed. In this work, the subject is viewed through the "theoretical eye" of a physicist, but the topics are treated in an elementary fashion. The main features of the presentation are its instructive, model-based approach; detailed step-by-step derivations of the equations; smooth progression through the simple wave-function matching techniques to the more...