The Aegis ballistic missile defense system, also known as Sea-Based Midcourse, is a Missile Defense Agency program under the United States Department of Defense developed to provide missile defense against short and intermediate-range ballistic missiles. The program is part of the United States national missile defense strategy and European NATO missile defense system.

The Missile Defense Agency (MDA) is a component of the United States government's Department of Defense responsible for developing a comprehensive defense against ballistic missiles. It had its origins in the Strategic Defense Initiative (SDI) which was established in 1983 by Ronald Reagan and which was headed by Lt. General James Alan Abrahamson. Under the Strategic Defense Initiative's Innovative Sciences and Technology Office headed by physicist and engineer Dr. James Ionson, the investment was predominantly made in basic research at national laboratories, universities, and in industry. These programs have continued to be key sources of funding for top research scientists in the fields of high-energy physics, advanced materials, nuclear research, supercomputing/computation, and many other critical science and engineering disciplines—funding which indirectly supports other research work by top scientists, and which was most politically viable to fund from appropriations for national defense. It was renamed the Ballistic Missile Defense Organization in 1993, and then renamed the Missile Defense Agency in 2002. The current director is U.S. Navy Vice Admiral Jon A. Hill.