Underpotential deposition (UPD), the phenomenon of metal monolayer(s) formation on a foreign metal substrate at a potential more positive than the equilibrium potential for bulk electro-deposition, has been the subject of considerable research in recent years because it yields model systems for investigation of the electrode/electrolyte interface. Broadly speaking, the defined sedimentary elements or substrates of UPD are not limited to metal elements, and also include other substances such as nonmetallic elements, nano-particles, and so forth. In virtue of the great potential that monolayer film of dissimilar metals can significantly change the potential distribution near the interface, the orientation of the solvent molecules and the adsorption behaviors and reaction ability of the base metal surface, UPD draws great academic attention to its applications in electro-catalysis as well as electro-deposition of metal and alloy. A large number of experimental reports about UPD have been published so far. Based on literature in existence, several theoretical models and the behavior characteristics of UPD were summarized. Moreover, some factors such as substrates, ionic adsorption and temperature on the UPD process were emphasized. Finally, its practical applications were highlighted and the future research directions were pointed out.

Contents
1 Introduction
2 Theoretical models for UPD
2.1 Relations between UPD potential shifts and work functions
2.2 Thermodynamic model
2.3 DFT method
2.4 Monte Carlo method
3 The processes of UPD
3.1 Nucleation characteristics
3.2 Influence of substrate
3.3 Influence of adsorption ions
3.4 Effect of temperature
4 Applications
4.1 Improve electrocatalytic activity
4.2 EC-ALE technology
4.3 Self-assembled monolayer
5 Conclusion and outlook