A many-electron model is proposed to describe the electronic structure of metal–semiconductor interfaces and semiconductor heterojunctions. The model is utilized to calculate the self-consistent one-electron potential in the vicinity of the interface. The boundary conditions ensuring thermal, mechanical, and electron-transfer equilibrium are imposed explicitly. Numerical calculations predict the validity of Schottky’s phenomenological boundary condition for metal–semiconductor contacts, the applicability of the electron affinity rule for semiconductor heterojunctions, and systematic correlations between Schottky barrier heights and heterojunction energy band offsets. The latter three results emerge as consequences of the rigorous and complete imposition of thermodynamic equilibrium boundary conditions and of the use of a model of the ion-core charge density in which atomic reconstructions at the interface relative to the vacuum surfaces are not considered explicitly.