Interpretations are given for the core-level binding energies, BE's, of the surface and bulk atoms of metals; in particular, the origin of possible differences, or shifts, between these BE's is considered. The mechanisms which are responsible for the BE shifts are identified and characterized through theoretical analyses of the surface electronic structure. The dominant mechanisms for the BE shifts are those which lead to the initial-state changes in the orbital energies and, hence, the Koopmans' Theorem BE's. This is particularly important because it means that the shifts can be directly related to the chemistry and physics of the systems. The surface electronic structure is described with molecular-orbital wavefunctions for cluster models of the surface; this theoretical approach is particularly well suited for the study of local chemical interactions. The different mechanisms act in a cancelling fashion which leads to rather small net shifts between the BE's of bulk and surface atoms. The mechanisms which explain the surface and bulk core-level BE shifts are rather general and may provide a basis for understanding other cases of core-level BE shifts.
