The chemical and catalytic properties of supported alloyed metal nanoparticles are reviewed. After describing the theoretical methods commonly used to predict the electronic structure of nanoalloys and the experimental methods used to characterize their geometrical structure, we focus on the catalytic properties of these systems and the opportunities that alloying provide to modulate them. Prototypical processes of key industrial relevance are used as test examples, including catalytic pollution control as three-way catalysts and NOx decomposition, the Fischer–Tropsch process, reforming of alkanes, electrocatalysis, and new energy resources as fuel cells and hydrogen storage. A section discussing how to bridge the gap between model studies employing surface science methods and technological heterogeneous catalytic systems concludes the chapter.