As we strive towards a sustainable energy economy, it becomes increasingly urgent to optimize electrochemical reaction rates at the practical device level. In heteroge-neous electrocatalysis, the reaction rate is a direct measure of the overall current which inexorably intertwines the kinetics of the catalytic surface reaction with mass transport. This crucial interplay manifests in a number of different phenomena including the diffusion-limited transfer of reactants to the active surface , ion crowding or so-called "local" pH effects , as well as the diffusion-controlled desorption/readsorption of surface-bound reaction intermediates . While surface kinetics are determined by atomistic processes over nanoscopic active sites, however, transport effects typically emerge over micrometer length scales. Bridging these two disparate scales into a (single) comprehensive reaction model is an ongoing, yet key, challenge both from a conceptual as well as a methodological point of view.