Bimetallic surfaces and catalysts and their chemical and catalytic properties have attracted considerable interest over the last several decades, both from fundamental reasons, but in particular because of the attractive catalytic properties of bimetallic catalysts, which were found to often exceed the performance of their constituents in terms of activity, selectivity and stability . The improved catalytic performance was attributed to an interplay of a number of different effects such as geometric ensemble effects, electronic ligand effects and electronic strain effects, where the first describes the influence of the size/configuration of active surface ensembles . Electronic ligand effects describe the effect of electronic modifications of the active site by different neighboring surface atoms . These as well as strain effects, which reflect the effect of lattice distortions as compared to the natural lattice of the respective surface layer on the electronic structure , can lead to changes in the adsorption energy of reactants and thus to significant modifications in the reaction kinetics . Finally, site-blocking effects, which reduce the number of active sites, have to be considered as well . Initially, these different effects were mainly concluded from changes in the catalytic activity upon varying the concentration of the respective components on bimetallic catalysts and polycrystalline bimetallic films . More detailed, atomic-scale insights were gained from Surface Science type model studies on structurally well-defined, single-crystalline bimetallic surfaces , which started in the late 1970s . Due to the rather well defined nature of these model surfaces, these studies allowed to more directly derive correlations between structural and electronic properties on the one hand and chemisorption and catalytic properties on the other hand. Nevertheless, despite the considerable progress in the atomic scale understanding of the chemistry of bimetallic surfaces, also these studies were limited in their predictive power because of missing information on structural and electronic details such as defect structure, atom distribution in bimetallic surface layers etc. This information became accessible only with the advent of modern highresolution spectroscopies and microscopies, in particular high-resolution scanning tunneling microscopy (STM) and, equally important, the development and application of powerful theoretical methods, in combination with the increasing knowledge in the reproducible preparation of structurally well-defined model surfaces / systems.