The first chapter of this book serves as a general introduction to energy storage and describes first the history and context of use. Especially the needs for storage are discussed in the context of changing from the centralized power generation of today to the distributed utility of tomorrow. Then, the structure of an electric storage system is presented, with its subunits as the power electronic conversion, the intermediary conversions, the storage form of energy, the control and diagnostic unit that supervises the storage device and estimates the state of charge. Further, general definitions are given, such as capacity, state of charge, losses, and efficiency.