Application fields of supercapacitors are expanding because they have a very large charge/discharge current density and a cycle durability of tens of thousands of cycles or more compared to secondary batteries. There are various kinds of supercapacitor: electric double layer capacitors with a relatively long history, pseudocapacitors that utilize electrochemical reactions, and the progress of hybrid capacitor technology that combines double layer capacity and electrochemical reactions. Development of electrode materials and electrolytes and new cell design for constructing devices support the performance improvement and expansion of new applied fields such as automobiles, heavy machinery, and energy harvesting. This book aims to provide engineers with the opportunity to review the latest information by integrating cutting-edge papers on science, technology, and the application of supercapacitors.