Studies of the electrodeposition dynamics of underpotential deposition (UPD) processes based on electrochemical and in-situ surface X-ray scattering studies are presented. The studies focus on the UPD of Hg on Au(111) in sulfate media, and Cu UPD on Pt(111) in sulfate and chloride media. In the first case it is shown that Hg UPD on Au(111) in sulfuric acid follows a progressive nucleation mechanism which involves various surface structures. In the case of Cu UPD on Pt(111), the process appears to follow an instantaneous nucleation mechanism in both sulfate and chloride media. Time-resolved surface X-ray scattering studies of Cu UPD on Pt(111) in the presence of chloride demonstrate that the electrochemical relaxation and the achievement of long range order can take place on significantly different time scales. Analysis of time-resolved surface X-ray scattering data allows for the study of the dynamics of island growth.