In this work we undertake a first principles study of the deposition of Copper on Au(111) and Au(100) surfaces from a density functional point of view: The analysis of the different energetic contributions to the so-called underpotential shift indicates that the deposition of Cu on any of these surfaces should occur at overpotentials. We conclude that some other factors must be taken into account to explain the experimental results, such as anion coadsorption. We also analyze the work function and the electronic density in the system.