The technology surrounding missiles that fly at five or more times the speed of sound is not mature. But several nations have either tested hypersonic missiles or intend to do so. The envisioned military value of hypersonic delivery systems lies in their ability to strike quickly over long distances while evading early-warning radars and ballistic missile defenses—but some proponents of the technology also argue that hypersonics could replace nuclear weapons in some roles. Critics, meanwhile, see hypersonic missiles as destabilizing: They could be mistaken for ballistic missiles, could be used in attacks against nuclear and other strategic military assets, and could even be outfitted with nuclear warheads. Amid these concerns, calls have emerged to halt a developing hypersonic arms race by enacting a moratorium on hypersonic testing and eventually establishing a test ban treaty. Below, experts from the United States, India, and China debate this question: How serious a threat to global security do hypersonic missiles represent, and is a test ban the best way to moderate such a threat?