The electrochemical capacitor (EC), also known as supercapacitor, is an energy storage device possessing a near infinite life-cycle and high power density recognised to store energy in the double-layer or through pseudocapacitance as a result of an applied potential. Fundamental principles of charge storage in relation to the important physical and chemical characteristics of electrode materials are addressed in the following review, with carbon-made electrodes, specifically activated carbon, carbon fibres and aerogels, carbon nanotubes and graphene emphasised in regards to their enhancement of the characteristic energy and power densities of ECs. Pseudocapacitive materials, notably transition metal oxides and nitrides, and conducting polymers are remarked by the potential to further improve EC performance through synergistic effects and asymmetric design. Research towards gaining a better understanding of charge storage in sub-micropores, material design and improving the performance of alternative electrolytes are expected to greatly enhance the capabilities of these devices in the near future.