This POSTnote looks at hypersonic missile technologies, efforts to develop them, potential applications, and the possible challenges they may present for missile defence and global stability. Key Points: • Hypersonic missiles combine speeds of over five times the speed of sound with significant manoeuvrability during flight. • Their manoeuvrability enables them to change trajectory during flight, making their flight-path and target difficult to predict. • They fly at lower altitudes than ballistic missiles, which means that they may be harder to track at long distances with some surface-based sensors, such as certain radar. • There are two main types of hypersonic missile: hypersonic glide vehicles (HGVs) and hypersonic cruise missiles (HCMs). • HGVs are mounted onto rocket boosters for launch and may be accelerated to speeds of Mach 20 or more. The glider then separates from the booster and flies unpowered in the Earth’s upper atmosphere at altitudes of 30-80 km, before diving towards the target. • HCMs typically have a ramjet or scramjet engine that enables them to reach hypersonic speeds at altitudes of 20-40 km. • China and Russia have reportedly deployed hypersonic missiles that could deliver conventional or nuclear weapons. The US is testing multiple hypersonic technologies. • The AUKUS agreement between the UK, US and Australia includes developing hyper-sonic and counter-hypersonic technologies. • Developing hypersonic missiles requires significant research and development challenges to be overcome, contributing to their high development and manufacturing costs. • The speed, manoeuvrability and altitude of hypersonic missiles may challenge existing missile defences, although their uses and effectiveness are still being assessed. • Defence analysts disagree about the potential implications of hypersonic missiles for global peace and stability. Some suggest they could increase the risk of conflict escalation, while others say that they will not alter the strategic balance between nuclear powers. • Arms control, export controls and other measures may help limit potential harm to peace and stability, but these approaches face challenges.