A flywheel energy storage (FES) system is an electricity storage technology under the category of mechanical energy storage (MES) systems that is most appropriate for small- and medium-scale uses and shorter period applications. In an FES system, the surplus electricity is stored in a high rotational velocity disk-shaped flywheel. The stored energy in the form of kinetic energy will be later used to drive a generator and thereby produce electrical power. This chapter gives a fundamental understanding of the flywheel and FES system mechanism followed by a history of the development of this technology in the literature and practice over recent years. It also discusses FES system applications in several fields of science and technology, advantages and disadvantages, design concept, and components. Finally, it presents the mathematical formulation governing the system as well as the future perspectives of the technology.