 Flywheel energy storage systems, FES, are a promising technology for providing energy storage solutions for electrical grids. These systems have several advantages over other energy storage technologies such as high cycle life, long operational life, high power density, low environmental impact, and the ability to store large amounts of energy without any upper limits. This paper provides a comprehensive review of FES, including their main components and applications, which has not been covered in previous reviews. Additionally, it discusses the latest developments in the field, including new designs and commercial prototypes. This article was authored by Mustafa E. Amir and Keith Arpulin.