Flywheels are one of the oldest and most popular energy storage media owing to the simplicity of storing kinetic energy in a rotating mass. Flywheel energy storage systems (FESSs) can be used in different applications, for example, electric utilities and transportation. With the development of new technologies in the field of composite materials and magnetic bearings, higher energy densities are allowed in the design of flywheels. The amount of stored energy in FESS depends on the mass and the speed of the rotor, whereas the maximum power that can be generated depends on the rating of the electric machine and the power electronics-based converters. The construction (electric machine, power electronics converters, bearings, and casing), features (efficiency, lifetime, cost, and safety), and applications (utility, transportation, and renewable energy sources) represent the important aspects in understanding FESSs. Wiley Online library