In general terms, understanding and mastering the physics and chemistry of adsorption processes at nanostructures and surfaces is a basic requirement for the full development of nanoscience and nano-technology. Metal surfaces are effective chemical agents capable of adsorbing and/or dissociating molecules impinging from the gas phase. Industrial processes of enormous economical impact, such as corrosion and heterogeneous catalysis, greatly benefit from recent developments in basic research on this matter. Over the last years, the combination of experimental molecular-beam techniques and refined theoretical calculations based on ab-initio methods have led research on the field to a new stage, in which detailed investigations of the kinetics and dynamics of molecular reactivity at surfaces are possible.