The field of heterogeneous catalysis is built upon the exploitation of materials structured on the nanometre-scale. Supported, size-selected nanoclusters open up new opportunities for research in catalysis as the catalyst functionalities can be explored as a function of cluster size, composition and atomic structure. In this review, we consider as an example, the catalytic oxidation of carbon monoxide by oxidised palladium clusters, coupling vibrational infrared spectroscopy with temperature programmed reaction experiments. The results clearly show that the detailed atomic structure of the clusters plays a crucial role in the catalytic performance. The new generation of aberration-corrected electron microscopes provides a new window on the structure of such nanoparticles, with atomic precision, as illustrated by High-Angle Annular Dark-Field detector-Scanning Transmission Electron Microscopy images of size-selected gold and palladium clusters.