New electrochemical application using room-temperature ionic liquids (ILs) are introduced, such as lithium secondary batteries, electrochemical double layer capacitors, and novel types of electrical devices for sustainable and renewal energy society. ILs have so many combinations, owing to many cation/anion species. In this chapter, we introduce properties from fundamental (general and special physicochemical properties) to electrochemical applications of ILs. We also discuss importance of molecular design and application target of ILs.