Recent developments in the theory of underpotential deposition (upd) are analysed. After an introductory comment on the thermodynamics of upd, the application of density functional theory to this phenomenon is reviewed. The principles and predictions of other model types are also discussed. Among these, we analyse models based on first principles calculations, statistical thermodynamics, the Newns-Andersons model for adsorption and semi-empirical quantum mechanical calculations.