Designing well-defined nanoparticles and nanoalloys is a tremendous way to achieve in-depth understanding of their intrinsic properties. In particular, structure and composition of the core and the surface of nanoalloys can be investigated by a combination of state-of-the-art in situ microscopy and spectroscopy. These nanoalloys represent a playground to establish structure-properties relationships within the nano-matter. They provide a much needed understanding of the distribution of each element within the nanoparticles, depending on the environment (gaseous atmosphere, temperature, etc.). This distribution may evolve over time. Lighter elements, such as phosphorus, are critical to the reactivity of the nanoparticle's surface in reactions such as CO or CO 2 hydrogenation. Here, the rational design of nanoalloys will be discussed (reactants choice, composition control), in relation with their surface state. Consequences on heterogeneous and homogeneous catalytic reactions, as well as for energy storage and conversion, will be illustrated through examples.