The description of the electronic structure of an interface between two materials is one of the main goals of solid state theory. In the case of a metal-semiconductor interface a basic problem is the calculation of Schottky barriers. A review is presented in which different theories on the formation of the Schottky barrier are discussed. The role of the interface states and their physical origin are discussed. Particular attention will be paid to the question whether the barrier heights are influenced by the details of the interface structure.