Recent synthesis developments have refined the use of catalyst alloys, creating structures consisting of an active shell surrounding an inert core. These have been remarkably successful in reducing the total metal costs while also enhancing the surface activity of the shell. To reach an atomic-scale understanding of their structure and chemistry, Field ion/field electron microscopy (FIM/FEM) and atom probe tomography (APT) are uniquely suited to this task. This chapter will review the highlights of the research aiming at characterizing the surface reactivity, the three-dimensional structure and the composition of nano-engineered solid catalysts, as well as discussing ongoing experimental developments to improve the specimen forms and to construct in situ reaction cells, both designed to allow experiments to mimic the structures and operating conditions of industrial catalysts as closely as possible.