Applications of synchrotron-x-ray-scattering techniques for the investigation of electrochemical interphases are reviewed in this article. The unique characteristics of x-ray-scattering technique that make possible the in-situ structural examination of solid interfaces buried under a layer of solution are discussed. A basic description of the theory and practice of x-ray-scattering techniques is given, followed by an elementary discussion of the theory of scattering from surfaces and interfaces. We then describe some special experimental aspects arising in the investigation of electrochemical interphases. Finally, recent, characteristic examples are reviewed, such as, structural studies of under-potential-deposited monolayers, studies of surface reconstruction of gold single crystals under electrochemical potential control, electrochemical oxidation and reduction studies of copper films, and anodic oxidation induced roughening of platinum single-crystal surfaces.