Much of our physical understanding of the electrostatic properties of solid-liquid interfaces derives from the pioneering developments of Gouy, Chapman, Stern, and Grahame, over 100 years ago. These pioneers popularized the concept of the electric double-layer (EDL), which describes the microscopic structure of the space-charge region, i.e., ρ(z), that spontaneously forms at the liquid interface. This concept and the models that describe the EDL have been essential in our ongoing efforts to interpret a wide array of electrochemical measurements and in generating predictions for how chemical processes can be controlled through changes in applied potential or electrolyte composition. The EDL structure prescribes the spatial variations in the interfacial charge density and thus determines the shape of the interfacial potential profile and its associated electric fields. EDL models predict experimentally measurable properties, such as the interfacial differential capacitance, 64-66 and thus can be validated or parameterized through experimental feedback. 65 However, many of the specific details about EDL structure cannot yet be uniquely determined. Accordingly, uncertainty remains about which EDL model is "correct" or appropriate for interpreting a given experiment.