  Interface science has become a key aspect for fundamental research questions
and for the understanding, design and optimization of urgently needed energy
and information technologies. As the interface properties change during
operation, e.g. under applied electrochemical stimulus, and because multiple
bulk and interface processes coexist and compete, detailed operando
characterization is needed. In this perspective, I present an overview of the
state-of-the art and challenges in selected X-ray spectroscopic techniques,
concluding that among others, interface-sensitivity remains a major concern in
the available techniques. I propose and discuss a new method to extract
interface-information from nominally bulk sensitive techniques, and critically
evaluate the selection of X-ray energies for the recently developed meniscus
X-ray photoelectron spectroscopy, a promising operando tool to characterize the
solid-liquid interface. I expect that these advancements along with further
developments in time and spatial resolution will expand our ability to probe
the interface electronic and molecular structure with sub-nm depth and complete
our understanding of charge transfer processes during operation.
