An electric double-layer capacitor is one of the energy storage devices, which consists of carbon polarizable electrodes and electrolytic solution. It has been used for a memory back-up device since 1978 and recently advanced capacitors with higher capacitance and lower resistance are developed for high-rate discharge uses. In the paper, outline of the capacitors, development history and present status of the capacitors, effective factors to capacitor characteristics, and potential application of capacitors are discussed. Electrical characteristics of the newly developed capacitor for power uses, “UP-Cap”, are described. Future prospect of the capacitors as energy storage devices will be discussed.