In this chapter considers the storage of energy in capacitors. Capacitors can be traditional dielectric capacitors, electrolytic capacitors or electrochemical capacitors. While dielectric and electrolytic capacitors have a high power density, they have a very low energy density. Supercapacitors, have a higher energy density but lower power density than traditional capacitors and fill the gap between capacitors and batteries for electrical energy storage devices. The present chapter provides an overview of the properties of capacitors and supercapacitors and a survey of their applications.