Recent theories developed to explain the underpotential deposition (UPD) phenomenon and several relevant thermodynamics derivations concerning UPD are reviewed. Models based on density functional theory are found to be useful to derive general trends. Semiempirical quantum mechanical calculations, first principle band structure calculations, and models based on statistical thermodynamics have been applied successfully to particular systems. Models for incommensurable adsorption and the application of other more general adsorption models to UPD phenomena are also discussed.