In the following, we will focus on the structural, electronic, chemical and catalytic properties of structurally well-defined, single-crystalline bimetallic model surfaces, where compared to mostly polycrystalline, technical surfaces the variety of different surface sites is significantly lower. This allows a quantitative determination and evaluation of the abundance of different structural elements (see next section), which is mandatory for an unambiguous interpretation of the results obtained from adsorption and catalysis studies.