Flywheels with composite rotors are being developed as energy storage devices to replace conventional chemical batteries in satellites and other space systems, and are currently being used in terrestrial systems. The main advantages of composite flywheel systems are their ability to provide high instantaneous power and a capacity for large energy storage over many charge/discharge cycles. Currently, an industry working group is in the process of developing a standard that can be used to certify a composite rotor for safe and reliable operation in space. To assist in the development of such a standard, a review of state-of-the-art composite rotor technologies has been conducted. The goal of the review was to provide a clear picture of composite flywheel rotor technologies, and identify areas that require further development. Technical areas covered in the review were: rotor design approaches; composite analysis methods; composite materials' static strength, fatigue, creep, and stress rupture data; spin test and other test methods; nondestructive evaluation (NDE) methods; in-service health monitoring techniques; and fracture control requirements. This paper summarizes the review and provides recommended research and development topics that are needed to fill the technology gap.