Heterogeneous catalysis is among the most important fields of application of (supported) metal nanoparticles, including nanoalloys. Controlling their chemical composition and distribution offers the possibility to finely tune the nature of the surface “active sites”, and thereby optimize the catalytic performances. Geometric and electronic effects have been classically proposed to rationalize these observations, though the actual drivers may be more intricate.1 In this lecture, the basic concepts of metal nanocatalysis will be introduced together with the specific case of nanoalloys. Several examples using advanced characterization techniques including operando spectroscopies2 will illustrate nanoalloying effects on catalyst performance, as well as catalyst environment effects on nanoalloy structure.3 Beyond the classical bimetallic alloys of late transition metals, appealing classes of nanoalloy catalysts such as intermetallic compound catalysts4 and single-atom alloy catalysts5 will also be evoked.