Molecular modelling and simulation will be the key to success in designing tomorrow's high-energy and high-power supercapacitors. These are power devices that can be fully charged/discharged in a few seconds. They show high energy density compared to capacitors, and they play an important role complementing batteries in the energy storage field. However there is a tremendous need for increasing the energy density of these devices, which can only be achieved by designing new materials. Electrochemical Double Layer Capacitors (EDLCs) is a family of supercapacitors, in which the charge is stored at the electrode/electrolyte interface through reversible ion adsorption at high-surface-area porous carbon electrode. A molecular-scale description of the electrode/electrolyte interface is a necessary step towards the design of new materials with optimized energy storage capabilities. Here I review the most recent theoretical techniques which have been used for studying these systems, and I show how they have enhanced our understanding of their performances.