In this chapter, a general view of metallic alloys nanoparticles from the atomic-scale view using advanced transmission electron microscopy techniques such as aberration-corrected electron microscopy (Cs-corrected) and high-resolution electron microscopy, is covered. It is well established that catalytic activity of metal nanoparticles is increased by roughness at the atomic scale. Indeed a flat smooth surface will be less active than its stepped counterpart. Metal nanoparticles have in most cases facets with different Miller indexes. However, the control of the shape and crystal structure is not easy to achieve. The possibility of passivizing the nanoparticle surface with organic molecules is a way to control the nanoparticle shape. Using separation techniques such as electrophoresis, it is possible to obtain a batch of particles with the same mass in average and hence the same number of atoms. Passivized particles still have enough free sites to allow catalytic activity. Then, it is possible by using aberration-corrected electron microscopy to obtain the structure and surface structure of nanoparticles. In this work, we will report those methods and demonstrate its application for mono- and multimetallic clusters. The surface of the metallic nanoparticles plays a crucial role in the catalytic activity and interaction between elements in the surface and in the core of the core-shell structures. In this way, the analysis of chemical composition and chemical bonding needs to be obtained using analytic techniques such as X-ray energy-dispersive spectroscopy and electron energy-loss spectroscopy. Herein, we also present quantitative analysis of particles with few atoms by atomic counting using a statistical method applied in Cs-corrected high-angle annular dark-field scanning transmission electron microscopy images.