  Key technologies in energy conversion and storage, sensing and chemical
synthesis rely on a detailed knowledge about charge transfer processes at
electrified solid-liquid interfaces. However, these interfaces continuously
evolve as a function of applied potentials, ionic concentrations and time. We
therefore need to characterize chemical composition, atomic arrangement and
electronic structure of both the liquid and the solid side of the interface
under operating conditions. In this chapter, we discuss the state-of-the-art
X-ray based spectroscopy and diffraction approaches for such 'operando'
characterization. We highlight recent examples from literature and demonstrate
how X-ray absorption spectroscopy, X-ray photoelectron spectroscopy and surface
X-ray diffraction can reveal the required interface-sensitive information.
