Thermodynamically stable (see below) monometallic films (A) on a metal support (B) can be formed if the metal A does not form solid solutions in the bulk of B. In many cases, however, the confinement to the top layer(s) is reached by kinetic limitations, which kinetically stabilize the film against bulk dissolution. The same is true also for surface alloys on top of a monometallic substrate. For surfaces of bulk alloys, the composition of the surface region may differ significantly from that of the bulk composition, depending on the experimental conditions. Here it is important to note that essentially all of these model film systems with typically > 10 6 metal layers in the substrate and up to a few atomic layers in the film are only metastable at temperatures reached during preparation of such model system, due to the extreme dilution. Intermixing is thermodynamically favored from entropic reasons, and the films are only stabilized by kinetic limitations against bulk dissolution Changing from massive bulk samples to metal nanoparticles, the different types of film systems are often denoted as core-shell systems, where core and shell consist of different mono-or bimetallic phases.