Nanoparticles of alloys represent a very interesting subject for fundamental research and, at the same time, a lot of useful applications in industrial catalysis, microelectronics, sensors and medicine. Their properties depend on the atomic and chemical structure which has been the subject of intensive researches. This paper presents some trends in theoretical predictions of the chemical ordering and surface segregation in Pt-based nanosized systems, especially transition and noble metals for which catalytic, magnetic and optical properties are well known. By associating two different metals, it is possible to improve selectivity in catalysis, or to increase magnetic anisotropy in magnetic systems, or to tune the surface plasmonic resonance in optical absorption, but the question is how the two species will mix or demix and how they will be distributed on the surface and in the core of the nanoparticle. This paper will give some general overviews concerning Pt–X, X=Co, Pd or Ag systems obtained from atomistic simulations. It will make a link between the chemical configuration adopted by the nanoalloys and the chemical characteristics of the system in terms of ordering tendency in bulk alloys and surface segregation in surface alloys.