The application of modern “surface science” experimental techniques to probe free semiconductor surfaces and their interfaces with gases and metals has underlined the important role played by surface imperfections in a range of processes. In this article we consider the role of surface defects in the pinning of the Fermi level at free surfaces and in the adsorption and oxidation mechanisms involved when surfaces interact with gases and vapours. In addition we consider in detail the role of imperfections at metal-semiconductor interfaces and the part they play in chemical interactions and in determining electrical characteristics such as Schottky barrier height. It is shown that there is a great deal of conflicting views in the literature and many of the unresolved questions are discussed.