The feasibility of applying the double-layer capacitor to uninterruptible power systems and electric vehicles is addressed. The double-layer capacitor improves energy storage density by two orders of magnitude over the traditional electrolytic capacitors. Compared to batteries, the energy density of the double-layer capacitor is lower, but its charge/discharge rate is significantly higher. The application of double-layer capacitors can be tailored to auxiliary energy and burst mode storages which require fast charge/discharging and high energy density. A description is given of the double-layer capacitor technologies including its materials, structure, and characteristics; potential application areas of double-layer capacitors; and analytical and experimental results of double-layer capacitor applications. >