The Aegis Ballistic Missile Defense System (Aegis BMD or ABMD) is a United States Department of Defense Missile Defense Agency program developed to provide missile defense against short to intermediate-range ballistic missiles. It is part of the United States national missile defense strategy. Aegis BMD (also known as Sea-Based Midcourse) is designed to intercept ballistic missiles post-boost phase and prior to reentry.
Aegis is now used also by the Japan Maritime Self-Defense Force, Spanish Navy, Royal Norwegian Navy, and Republic of Korea Navy. Over 100 Aegis-equipped ships have been deployed in five navies worldwide. The Royal Australian Navy selected the Aegis system for placement on its new Air Warfare Destroyers, and it is part of NATO's European missile defence system.