Recently, much attention has been devoted to the development of synthetic approaches that can exert facile control over the size, shape, architecture and composition of transition metal nanoparticles (TMNPs).  The advent of such approaches could enable the rational design of nanoelectrocatalysts, as the catalytic properties of TMNPs are highly sensitive to their morphologies and elemental compositions.  One strategy that is actively being investigated involves the underpotential deposition (UPD) of transition metals onto the surfaces of seed TMNPs.  UPD-based approaches for shape control are attractive since the deposition process is surface-sensitive and is easily controlled by varying the synthesis conditions, such as the transition metal salt concentration and the pH of the electrolyte.  Nevertheless, the ability to predict the equilibrium shape of UPD-modified TMNPs is challenging because of difficulties in modeling the UPD process in electrolytic media.  Therefore, in order to develop a strong connection between the synthesis conditions and the equilibrium shapes of UPD-modified TMNPs, better theoretical descriptions of UPD phenomena are required. In this work, we report progress on the first-principles modeling of UPD phenomena under realistic electrochemical conditions.  Key to this model is the ability to include the influence of solvent dipoles and the electrical double layer along the solid–liquid interface. We capture these effects through a recently released solid–solution interface model.  Employing this approach, we are able to accurately calculate the measurable shift in redox potential associated with the UPD process on planar surfaces, as well as the adsorption isotherms including entropic effects.  The development of this predictive first-principles UPD model and extensions to modeling equilibrium TMNP shapes are discussed.