The Phalanx CIWS (pronounced sea-wiz) is a close-in weapon system for defense against incoming threats such as small boats, surface torpedoes, anti-ship missiles and helicopters. It was designed and manufactured by the General Dynamics Corporation, Pomona Division,[3] later a part of Raytheon. Consisting of a radar-guided 20 mm (0.8 in) Vulcan cannon mounted on a swiveling base, the Phalanx has been used by the United States Navy and the naval forces of 15 other countries. The US Navy deploys it on every class of surface combat ship, except the Zumwalt-class destroyer and San Antonio-class amphibious transport dock.[5] Other users include the British Royal Navy, the Royal Australian Navy, the Royal New Zealand Navy, the Royal Canadian Navy and the US Coast Guard (aboard its Hamilton- and Legend-class cutters).

A land variant, known as the LPWS (Land Phalanx Weapon System), part of the C-RAM system, has recently[when?] been deployed in a short range missile defense role, to counter incoming rockets, artillery and mortar fire.[6][7][8] The U.S. Navy also fields the SeaRAM system, which pairs the RIM-116 Rolling Airframe Missile with sensors based on the Phalanx.

Because of their distinctive barrel-shaped radome and their automated nature of operation, Phalanx CIWS units are sometimes nicknamed R2-D2 after the famous droid character from the Star Wars films.[