Members of Congress and Pentagon officials have placed a growing emphasis on U.S. programs to develop hypersonic weapons as a part of an effort to acquire the capability for the United States to launch attacks against targets around the world in under an hour. Hypersonic weapons can travel faster than Mach 5, or about 1 mile to 5 miles per second. This interest is driven by both the perceived mission need for conventional prompt strike systems and concerns about falling behind Russia and China in the development of these technologies. The United States is pursuing two key technologies for this purpose: boost-glide systems that place a hypersonic glider atop a ballistic missile booster or shorter range rocket systems, and hypersonic cruise missiles that would use scramjet technologies. This report focuses, primarily, on the Pentagon's ongoing program to develop ballistic missile-based conventional prompt strike systems. This effort has been underway for about 15 years. The George W. Bush Administration demonstrated an interest in the use of conventional weapons for precision, long-range strike missions in the 2001 Nuclear Posture Review (NPR). This study called for the integration of precision conventional weapons with strategic nuclear forces in a new category of "offensive strike" weapons. Several other Pentagon studies published during the Bush Administration also called on the United States to develop the capability to attack targets around the world, in under an hour, with conventional warheads. The Obama Administration, in the 2010 NPR, also emphasized the role that long-range, non-nuclear systems could play in supporting "U.S. regional deterrence and reassurance goals." The 2010 NPR indicated that conventional power projection capabilities were part of "effective regional security architectures," arguing that these capabilities could help the United States assure and defend its allies, while reducing the role of nuclear weapons in U.S. security strategy. The Trump Administration's Nuclear Posture Review did not address the role of advanced conventional weapons in U.S. nuclear posture, but the Administration has shown continuing support for the development of long-range precision conventional weapons in its budget request for FY2019. It has also supported a joint Air Force/Navy/Army effort to develop and deploy hypersonic capabilities by the early 2020s. Moreover, Michael Griffin, the Undersecretary of Defense for Research and Engineering, has spoken often about the challenges presented by Russian and Chinese hypersonic weapons programs and about the need for the United States to accelerate its efforts. In testimony before Congress, he noted that the United States does not "have systems which can hold them at risk in a corresponding manner, and we don't have defenses against those systems." He said, "It is among my very highest priorities to erase that disadvantage, creating our own systems to hold them at risk and to provide defense." In 2003, the Department of Defense (DOD) specifically identified a new mission—prompt global strike (PGS)—that sought to provide the United States with the ability to strike targets anywhere on Earth with conventional weapons in as little as an hour, without relying on forward based forces. DOD argued that this capability would bolster U.S. efforts to deter and defeat adversaries by providing the United States with the ability to attack high-value targets or "fleeting targets" that might be visible for only a short amount of time promptly, at the start of or during a conflict. DOD has considered a number of systems that might provide the United States with long-range strike capabilities. These include bombers, cruise missiles, ballistic missiles, and boost-glide technologies that would mate a rocket booster with a hypersonic glide vehicle. During the George W. Bush Administration, analysts began to note that long-range ballistic missiles armed with conventional warheads would be an ideal weapon for this mission. They argued that these weapons, based in the United States or on submarines at sea, could attack targets worldwide with a high degree of precision in a short amount of time. However, during the latter years of the Bush Administration and the early years of the Obama Administration, DOD's programs began to focus on hypersonic weapons as the technology of choice for this mission. In addition, in 2012, the Pentagon changed the focus of the program from "prompt global strike" to "prompt strike," which allowed the program to consider the deployment of a hypersonic glider on a sea-based intermediate-range missile. The Pentagon is also conducting research, through DARPA and the Air Force, into shorter-range, air-delivered, hypersonic cruise missiles. Some analysts have questioned the need for these programs, raising concerns, for example, about the possibility that U.S. adversaries might misinterpret the launch of a missile with conventional warheads and conclude that the missiles carry nuclear weapons. They have also questioned whether existing U.S. military capabilities might meet the need for prompt, conventional attacks in most potential conflict scenarios without raising the risk of miscalculation or misunderstanding. Some have also questioned whether the United States should accelerate its hypersonic and prompt strike programs in response to Russian and Chinese hypersonic weapons programs. Some argue that the United States should not only focus on maintaining a technological edge in this area, but should also counter Russian and Chinese hypersonic weapons with similar U.S. weapons. Others, however, argue that an "arms race" in hypersonic weapons could be destabilizing, as the weapons' speed would shorten attack and response times and could possibly prompt early use during a crisis. Congress has generally supported the rationale for the prompt strike mission. In the past, it has restricted funding and suggested changes in the direction of specific programs, but, recently, it has pressed the Pentagon to place a higher priority on the program and to accelerate its development. Congress is likely to continue to review the technologies and programs related to this mission as a part of the annual authorization and appropriations process. This report provides an overview of the rationale for the PGS/prompt strike mission and the possible deployment of conventional warheads on long-range ballistic missiles or boost-glide systems in support of this mission. It then reviews the Air Force and Navy efforts to develop these systems. It summarizes congressional reaction to these proposals, and provides a more detailed account of the issues raised by these concepts and programs. Throughout the Cold War, the United States maintained military bases overseas so that it could position its troops to deter, and if necessary, respond promptly to an attack from the Soviet Union or its allies. These forward bases were located, for the most part, in Europe and Asia—regions where conflict seemed most likely to occur. These overseas bases and forces were believed not only to increase preparedness, but also to deter conflict by their very presence in unstable regions. However, with the demise of the Soviet Union and the end of the Cold War, analysts argued that the United States must be prepared to fight in unexpected areas against a wide range of potential adversaries who may possess a great variety of military capabilities. Although the United States continues to deploy its military forces at bases around the world, it has begun to restructure, and, in many cases, reduce, its forces based overseas. It has also sought to improve its ability to move military forces into a region quickly when and if a conflict occurs. Moreover, as some observers have noted, the United States can no longer be certain that these bases are located close to the most likely areas of conflict. As a result, many analysts and military officials have argued that the United States must maintain and enhance its long-range strike capability so that it can strike anywhere in the world with forces that are based in or near the United States, or with forces that have the range to reach targets across the globe from wherever they are deployed. This would not only allow the United States to pursue an adversary without relying on forward bases, it would also allow the United States to reach targets deep inside an enemy's territory if that area were out of the range of U.S. forces deployed at bases or on naval forces in the region. Moreover, if an adversary developed air defenses or other capabilities that could deny U.S. aircraft access to critical targets, a long-range strike capability based on ballistic missile technologies could prove valuable if launched early, as a "leading-edge" capability that degraded an opponent's defenses. Analysts argue that these types of systems would be far less sensitive to an adversary's anti-access and area denial (A2/AD) efforts. Further, some analysts argue that the United States must be able to attack targets across the globe in a matter of hours or less, either at the start of a conflict or during ongoing operations. This is because U.S. adversaries might adapt to the U.S. precision-strike capability by denying targeting information with concealment techniques or mobility, leaving the United States with little time to attack after it identified relevant targets. Moreover, many have noted that adversaries could seek to protect their assets by deploying them in buried or hardened facilities, leading to a requirement for improvements in the U.S. ability to defeat hardened and deeply buried targets promptly, before the adversary employed the hidden capabilities. The need for prompt long-range, or global, strike capabilities has been addressed in general defense policy studies, such as the 2001, 2006, and 2010 Quadrennial Defense Review (QDR) Reports. The 2001 QDR noted that the U.S. defense strategy "rests on the assumption that U.S. forces have the ability to project power worldwide." The 2006 QDR expanded on the need for prompt global strike capabilities, noting that they would provide the United States with the ability "to attack fixed, hard and deeply buried, mobile and relocatable targets with improved accuracy anywhere in the world promptly upon the President's order." This QDR went on to call for the deployment of a prompt global strike capability, using Trident submarine-based ballistic missiles armed with conventional warheads, within two to four years.