In policy pronouncements over the last two administrations, the protection of the American homeland was regularly identified as the first priority of U.S. missile defense efforts. Homeland missile defense today is provided by the Ground-based Midcourse Defense program and other elements of the larger Ballistic Missile Defense System. The defenses fielded today have advanced considerably since limited defensive operations began in late 2004, but nevertheless remain too limited, too modest, relative to emerging threats. The Missile Defense Agency’s path to improve the system may require additional effort to stay ahead of even limited missile threats. This report explains how the current system works as well as current and potential plans to modernize the system, and offers recommendations for its future evolution.