Image-potential states have been investigated widely by time-resolved two-photon photoemission (2PPE) as a model system to study electron dynamics at metal surfaces. Rare-gas layers, which represent a prototype for dielectric overlayers, can modify the coupling of these states to the bulk metal in a controlled way. It will be shown that binding energies and lifetimes for inelastic decay depend in a straightforward way on the electron affinity of the rare gas and the layer thickness. Many of the experimental results can be accounted for in a simple one-dimensional description with a potential that only varies perpendicular to the surface.For rare-gas layers of sufficient thickness new electronic states arise from the screened image-potential of the metal within the adlayer. These states have similar properties as the image-potential states on clean surfaces, but are spatially located at the dielectric/metal interface and can have energies above the vacuum level. We discuss the origin and the basic properties of these previously unexplored interface states for the system Ar/Cu(1 0 0). We will show how time-resolved 2PPE can be used to study their decay by resonant tunneling through the layer into vacuum and by electron–hole-pair decay into the metal.