The development of model catalyst systems for heterogeneous catalysis going beyond the metal single crystal approach, including phenomena involving the limited size of metal nanoparticles supported on oxide surfaces, as well as the electronic interaction through the oxide–metal interface, is exemplified on the basis of two case studies from the laboratory of the authors. In the first case study the reactivity of supported Pd nanoparticles is studied in comparison with Pd single crystals. The influence of carbon contaminants on the hydrogenation reaction of unsaturated hydrocarbons is discussed. Carbon contaminants are identified as a key parameter in those reactions as they control the surface and sub-surface concentration of hydrogen on and in the particles. In the second case study, scanning probe techniques are used to determine electronic and structural properties of supported Au particles as a function of the number of Au atoms in the particle. It is demonstrated how charge transfer between the support and the particle determines the shape of nanoparticles and a concept is developed that uses charge transfer control through dopants in the support to understand and design catalytically active materials.