I review a concept that models heterogeneous catalysis based on a surface-science approach. It is shown that models catching part of the complexity of the real system, which is connected with the finite size of active components and the flexibility of the arrangement of atoms in the active component, play an important part in determining the activity and selectivity of the system. I have chosen several examples from our own laboratory to elaborate the details and will put those into perspective with respect to the literature. I will show that Pd nanoparticles in hydrogenation incorporate hydrogen, which turns out to be crucial for the actual hydrogenation step. Another example correlates the structure of vanadia monolayer catalysts with its reactivity in methanol oxidation. With a third example we address the question of charge on Au nanoparticles when anchored to an oxide surface, a problem heavily discussed in the literature. Further examples refer to ultrathin oxide film catalysts in which the oxide metal interface controls either the charge state of Au particles grown on the film, and, in a last example, the oxide film itself exhibits remarkable CO-oxidation activity, which can be traced to a reactive intermediate structure of the ultrathin film.