The scanning tunneling microscope (STM) is today established as a unique tool for resolving the atomic-scale structure of surfaces. STM studies of model systems relevant to heterogeneous catalysis have made it possible address and resolve many important, fundamental questions. related to catalytic processes by imaging, in direct space, the atomic-scale structure of catalytically relevant model systems. e.g. adsorbate-covered single crystal surfaces or nanoclusters supported either on metals or oxide surfaces, These studies are normally carried out under well-controlled vacuum or pressure conditions. Here we discuss three recent STM studies of model catalyst systems, all illustrating flow the insight gained from fundamental studies of idealized model systems has been successfully linked to studies oil real catalyst systems operating under realistic conditions, and how this interplay has facilitated the development of new and superior high surface area catalysts. (c) 2005 Elsevier B.V. All rights reserved.