Two examples are presented of the application of in situ structure and composition sensitive techniques to elucidating the nature of the electrode-electrolyte interface. One case is the underpotentially deposited submonolayer of Cu on Au(111). Despite extensive electrochemical, LEED, RHEED, STM, AFM, and EXAFS investigations, the structure of the copper adlayer is still controversial. In situ surface EXAFS, surface X-ray scattering, and quartz crystal microbalance (QCM) data lead to the conclusion that the adlayer adopts a (root 3 x root 3)R30 degrees hexagonal honeycomb arrangement of Cu atoms in which the Cu coverage is 2/3 and the sulfate coverage 1/3. The other case is the structure of water at the Ag(111)-aqueous sodium fluoride interface. Analysis of X-ray scattering shows that the interfacial water molecules exhibit potential dependent layering and reorientation and a markedly increased density (twice bulk water) in the inner layer.