This monograph covers the rapidly developing field of electrochemical supercapacitors capable of exhibiting many Farads of capacitance per gram of active materials. The volume is aimed at a broad spectrum of scientists and technologists, including electrochemists, chemists, electrochemical and electrical engineers, and materials scientists. Hence the book is self-contained, starting with introductory chapters on the double-layer at interfaces, principles of electrode-process kinetics, and of electrostatics required in the treatment of double-layers and ion solvation, and elements of the theory of dielectrics. The main body of the material is concerned with procedures for characterizing the behavior and performance of electrochemical capacitors. Similarities and differences between electrochemical capacitors and batteries are treated in some detail, together with applications of both small and large capacitance units such as computer memory back-up and hybrid battery-capacitor systems for electric-vehicle drive-trains. A section is included on the practically important and scientifically interesting topic of self-discharge of capacitors and batteries, and the mechanisms that can be involved in that phenomenon. The work concludes with an up-to-date technology survey and a summary of recent and earlier patents in the field.