<div class="htmlview paragraph">Flywheel energy storage systems (FESSs), commonly know as flywheel batteries, are being developed for a number of power averaging applications in the transportation industry. Most FESS designs incorporate rotors made from composite materials for which operating experience is limited and little failure data is available. Given the high energy levels inherent in these devices, a major task for the FESS designer is to provide adequate containment structures to insure safe operation in the event of flywheel failure. This paper discusses ongoing work intended to provide a better understanding of composite flywheel failure mechanisms and development of a methodology for safe containment design.</div>