- We review recent analyses of the various aspects related to the performance of core/shell nanocatalyst particles used as electrodes in proton exchange membrane fuel cells. These nanoparticles usually consist of a thin layer of pure Pt in the shell and a core alloy made of a combination of metal elements that are targeted to meet two main objectives: reducing the catalyst price and enhancing the activity of the surface layer with respect to an equivalent particle made of pure Pt. Even though both objectives have been shown to be met, a huge challenge remains that is related to the long-term durability of the particle. This is because the less noble components are prone to relatively easy dissolution in the harsh acid conditions in which low-temperature fuel cells operate. The catalytic behavior of the nanoparticle towards the oxygen reduction reaction (ORR) and the evolution of the catalytic particle under this complex environment require a combination of experimental modern surface science and electrochemical techniques but also the formulation of models that allow a better understanding and a rational catalyst design. In this chapter, we review the state-of-the-art modeling of core/shell catalysts for the ORR. This involves various aspects that are intrinsic to the core/shell structure: surface segregation, metal dissolution, and catalytic activity. A number of methods ranging from ab initio density functional theory to classical molecular dynamics and Kinetic Monte Carlo are included in our discussion. Springer-Verlag London 2013.