The Aegis combat system, named for the mythological aegis shield, is a United States Navy weapons system . The acronym initially stood for Airborne Early warning Ground environment Interface Segment. The heart of the system is an advanced, automatic detect and track, multi-function phased array radar, the AN/SPY-1. This high-powered (four megawatt) radar is able to perform search, track and missile guidance functions simultaneously with a track capacity of over 100 targets. The first Engineering Development Model (EDM-1) was installed in the test ship, the USS Norton Sound in 1973.

The computer-based command and decision element is the core of the Aegis combat system. This interface makes the Aegis combat system capable of simultaneous operation against a multi-mission threat: anti-air, anti-surface and anti-submarine warfare. The Aegis system is being enhanced to act in a Theater Missile Defense role, to counter short- and medium-range ballistic missiles of the variety typically employed by rogue states; see Aegis Ballistic Missile Defense System.

The Navy built the first Aegis equipped cruisers using the hull and machinery designs of the Spruance-class destroyers. The first cruiser of this class was the Ticonderoga. The commissioning of the Bunker Hill opened a new era in surface warfare as the first Aegis ship outfitted with the Vertical Launching System (VLS), allowing greater missile selection, firepower and survivability. The improved AN/SPY-1B radar went to sea in the Princeton, ushering in another advance in Aegis capabilities. The Chosin introduced the AN/UYK-43/44 computers, which provide increased processing capabilities.

In 1980, a smaller ship was designed using an improved sea-keeping hull form, reduced infrared and radar cross-section and upgrades to the Aegis Combat System. The first ship of the DDG-51 class, the Arleigh Burke, was commissioned on the Fourth of July, 1991. The DDG-51 class was named after a living person, the legendary Admiral Arleigh Burke, the most famous American destroyerman of World War II.

DDG-51s were constructed in flights, allowing technological advances during construction. Flight II, introduced in FY 1992, incorporates improvements to the SPY radar and the Standard missile, active electronic countermeasures and communications. Flight IIA, introduced in FY 1994, added a helicopter hangar with one anti-submarine helicopter and one armed attack helicopter. The Aegis program has also projected reducing the cost of each Flight IIA ship by at least $30 million.

Aegis in other navies

• Japan operates four Kongo class destroyers of a modified Arleigh Burke design, with three more planned by 2010 as a result of Japan's fleet review.
• Spain is currently operating or building four Alvaro de Bazán class Aegis frigates, with at least two in commission.
• Norway is procuring five units of Spanish design and manufacture, as the Fridtjof Nansen class. The first unit of this type, the Fridtjof Nansen, was launched on June 3, 2004.
• South Korea is building Aegis variants of its KDX destroyers, called KDX-3 .
• Australia decided in November, 2003 to order three Aegis-equipped warships for theater missile defense (TMD), perhaps in connection with Collins class diesel-electric submarines for Taiwan. The Australian warships will be one of three foreign designs, though probably produced indigenously:
  • Early-build General Dynamics Bath Iron Works or Northrop Grumman Ship Systems Arleigh Burke class destroyers from U.S. Navy inventory. This would be expensive because the U.S. units are considerably larger and require more manpower than Australian specifications call for. The U.S. would acquire new units in lieu.
  • New-build destroyers based upon a modified and shrunken Arleigh Burke design.
  • IZAR's Alvaro de Bazán design (Spain). This would be somewhat increased in size due to increased size and weight requirements to accommodate TMD systems, as well as a newer Aegis variant. This design is a cooperation between Spain (F100 class), Netherlands (LCF class) and Germany (F124 Sachsen class) and based on Germany's Blohm + Voss GmbH highly successful MEKO design [1] This would be advantageous because of commonality with Australia's existing Anzac class frigates. This would have even greater sizing issues than Spain's F100. While spain's F100 class is using the Aegis radar system, Germany and The Netherlands decided to use the new developed european APAR (Active Phased Array Radar).

In all cases, Aegis integration would be performed by Lockheed Martin and the U.S. Navy.