Metal-semiconductor contacts are essentially of two kinds: ohmic contacts and Schottky barriers. The physics governing the formation of metal-semiconductor Schottky barriers-which are the basis of most electronic devices-and ways of tuning their height are reviewed. concentrating on the theory. It is shown that tuning can be achieved by modifying the interface geometry, by modifying the surface dangling bonds, or, in the case of heterojunction interfaces, by the addition of an interlayer. Directions for (experimental) future research are suggested.