Monometallic and bimetallic model catalysts on either refractory metal singlecrystals as planar surfaces or oxide supports as nano-size clusters have been systematically studied using X-ray photoemission spectroscopy (XPS), low energy ion scattering spectroscopy (LEIS), low energy electron diffraction (LEED), infrared reflection absorption spectroscopy (IRAS), and temperature programmed desorption (TPD) under ultra-high vacuum (UHV) conditions. Of particular interest in this investigation is the characterization of the surface composition, morphology, and electronic/geometric structure of the following catalysts: Au/TiOx, Au-Pd/Mo(110), Au- Pd/SiO2, Cu-Pd/Mo(110), and Sn/Pd(100). Structure-reactivity correlations during surface-alloy formation and adsorption-desorption processes were explained in terms of ensemble and ligand effects. Prospects of translating the accumulated atomic-level information into more efficient 'real world' catalysts were discussed.