Profound bias- and time-dependent changes in the electronic structure of graphene-based supercapacitor electrodes are demonstrated under operating conditions via a combination of in operando X-ray spectroscopy and ab initio modeling by J. R. I. Lee and co-workers on page 1512. The evolution in electronic structure reflects changes in the surface chemistry and morphology induced by polarization of the electrode-electrolyte interface and points to distinct pseudocapacitive and electric-double-layer capacitive channels for charge storage.