A review of the theory of clean metallic interfaces is presented. First, the contributions to the potential at a metallic interface due to bulk electron–electron, bulk electron–ion, and surface interactions, are illustrated for a metal–vacuum interface. We then review the description of this potential by phenomenological models and by dynamical models. Its description by a particular dynamical model, the local-density approximation for an electron fluid in an average positive background, is developed and compared with earlier models. The model is extended to describe bimetallic interfaces. Its major prediction is that the redistribution of charge at a bimetallic or metal semiconductor junction relative to the isolated metal–vacuum interfaces of its components causes substantial modifications of the potential at the interface. This effect suffices to describe the systematics of metal–semiconductor contacts without introducing surface states.