A model is presented which relates the diffusion behavior on metal surfaces to the charge density supplied by occupied two-dimensional free-electron surface states. Depopulation of these states by confinement onto small islands or by proper adsorbates increases the diffusion barrier on flat terraces and lowers the additional barrier for step-down diffusion, thus yielding enhanced interlayer transport. This provides a possible explanation for the phenomena of low-temperature “reentrant” and surfactant-mediated layer-by-layer growth. The model is supported by photoemission results.