Hypersonic missiles--specifically hypersonic glide vehicles and hypersonic cruise missiles--are a new class of threat because they are capable both of maneuvering and of flying faster than 5,000 kilometers per hour. These features enable such missiles to penetrate most missile defenses and to further compress the timelines for a response by a nation under attack. Hypersonic missiles are being developed by the United States, Russia, and China. Their proliferation beyond these three could result in other powers setting their strategic forces on hair-trigger states of readiness. And such proliferation could enable other powers to more credibly threaten attacks on major powers. The diffusion of hypersonic technology is under way in Europe, Japan, Australia, and India--with other nations beginning to explore such technology. Proliferation could cross multiple borders if hypersonic technology is offered on world markets. There is probably less than a decade available to substantially hinder the potential proliferation of hypersonic missiles and associated technologies. To this end, the report recommends that (1) the United States, Russia, and China should agree not to export complete hypersonic missile systems or their major components and (2) the broader international community should establish controls on a wider range of hypersonic missile hardware and technology.