Understanding the complex and inherently multi-scale interface between a charged electrode surface and a fluid electrolyte would inform design of more efficient and less costly electrochemical energy storage and conversion devices. Joint density-functional theory (JDFT) is an, in principle, exact theoretical framework which bridges the relevant length-scales by joining a fully ab initio description of the electrode with a highly efficient, yet atomically detailed classical DFT description of the liquid electrolyte structure. First, we introduce a universal approximate functional to couple any quantum- mechanical solute system with a classical DFT for any liquid and present classical density- functionals for both aqueous and non-aqueous fluids. This universal coupling functional predicts solvation energies of neutral molecules to within near-chemical accuracy of 1.5 kcal/mol and captures the qualitative and quantitative features of fluid correlation functions. We go on to explore the suitability of JDFT to describe electrochemical systems, reviewing the physics of the underlying fundamental electrochemical concepts and identifying the mapping between commonly measured electrochemical observables and microscopically computable quantities. We then introduce a simple, computationally efficient approximate functional which we find to be quite successful in capturing a priori basic electrochemical phenomena, includ- ing the capacitive Stern and diffusive Gouy-Chapman regions in the electrochemical double layer and potentials of zero charge for a series of metals. We also show that we are able to place our ab initio results directly on the scale associated with the Standard Hydrogen Electrode (SHE). Leveraging the above theoretical innovations, we then predict the voltagedependent structure and energetics of solvated ions at the interface between metal electrodes and an aqueous electrolyte, elucidating the origin of the nonlinear capacitance observed in electrochemical measurements. Finally, we discuss how JDFT calculations can determine the surface structure of a trained SrTiO3 surface under operating conditions for water-splitting and explore why this structure is correlated with higher activity than an untrained surface. We predict the specular X-ray crystal truncation rods for SrTiO3 , finding excellent agreement with experimental measurements from the Cornell High Energy Synchrotron Source (CHESS).