Publisher Summary Energy transmission is used not only to deliver energy from the sites of generation to the dominant sites of energy use, but also to deal with temporal mismatch between (renewable) energy generation and variations in demand. Therefore, energy transmission and energy storage may supplement each other. Mechanical devices are used to transfer mechanical energy over short distances, but mechanical connections with moving parts are not practical for distances of transfer that may be considered relevant for transmission lines. Storage of energy allows adjustment to variations in energy demand. The characteristics of an ideal energy storage system include availability for rapid access and versatility of the form in which energy from the store is delivered. The economic feasibility of energy storage depends on the relative fixed and variable costs of the different converter types and on the cost and availability of different fuels.