ThisFirst principles chapter analyzes recent literature studies, demonstrating the kinds of detailed mechanistic questions that can be addressed with modern theories that invoke surface chargingSurface charging and electrolyte response to attain desired electrode potentials. Theoretical calculations and experimental measurements are found to agree that underpotential deposited (upd) hydrogen atoms begin to form as the potential of a Pt electrode is decreased through the double layerDouble layer range to 0.35 V (RHE). As the potential is decreased from the double layer range, when 0.05 V is reached, coverage of H(ads) reaches 2/3 ML and H2 evolution commences. The question is addressed whether H(ads) can participate in reduction reactions. Theory shows the reaction energy for an R-H bond dissociating on the Pt surface is zero in the upd H range if sites for R(ads) and H(ads) are available.