The ballistic missile threat is increasing both quantitatively and qualitatively, and is likely to continue to do so over the next decade. Current global trends indicate that ballistic missile systems are becoming more flexible, mobile, survivable, reliable, and accurate, while also increasing in range. A number of states are also working to increase the protection of their ballistic missiles from pre-launch attack and to increase their effectiveness in penetrating missile defences. Several states are also developing nuclear, chemical, and/or biological warheads for their missiles. Such capabilities could be significant sources of military advantage during a conflict. But they may be equally significant in times of relative peace, when they undergird efforts to coerce states near and far. Regional actors such as North Korea and Iran continue to develop long-range missiles that will be threatening to the United States. There is some uncertainty about when and how this type of intercontinental ballistic missile (ICBM) threat to the U.S. homeland will mature, but there is no uncertainty about the existence of regional threats. They are clear and present. The threat from short- range, medium-range, and intermediate-range ballistic missiles (SRBMs, MRBMs, and IRBMs) in regions where the United States deploys forces and maintains security relationships is growing at a particularly rapid pace. This book provides an overview of select issues and policies of the ballistic missile defence program.