Electrochemical capacitors, also called supercapacitors, are unique devices exhibiting 20 to 200 times greater capacitance than conventional capacitors. The large capacitance exhibited by these systems has been demonstrated to arise from a combination of the double-layer capacitance and pseudocapacitance associated with surface redox-type reactions. The purpose of this review is to survey the published data of available electrode materials possessing high specific double-layer or pseudocapacitance and examine their reported performance data in relation to their theoretical expectations.