Our present understanding of the electronic structure of semiconductor surfaces is reviewed. It is shown that photoemission and inverse photoemission are ideal techniques for probing occupied and unoccupied electronic states, respectively. All quantum numbers of an electron can be determined, i.e., energy, momentum, spin and angular symmetries. For simple systems, such as clean ordered surfaces with a small unit cell it is possible to understand the electronic structure from first-principles calculations. For complex systems, such as encountered during oxidation and dry etching one is restricted to measuring the properties determined by short-range order. Core level spectroscopy with synchrotron radiation is able to determine the oxidation state and the local bonding of surface and interface atoms. © 1985 Springer-Verlag.