The understanding of the reactivity of supported nanometer-sized particles, which are used in heterogeneous catalysis, can be approached in two ways: by studying the reactivity of size selected and soft-landed small metal clusters containing 2 to 50 atoms ('molecular' approach) or by studying the reactivity of extended single crystal surfaces ('surface science' approach). We discuss the advantages and the limitations of these two approaches. We show in particular that often these two approaches cannot explain quantitatively the reaction kinetics for supported clusters a few nanometers in diameter. The peculiarity of these nanometer-sized supported clusters is due to their intrinsic heterogeneities: the presence of different types of facets, the presence of edges and the presence of the support. Taking account the recent experimental results on model supported catalysts, we show how the role of these heterogeneities can be investigated.