BAE Systems plc (BAE) is a British multinational arms, security, and aerospace company based in London, England. It is the largest defence contractor in Europe, and ranked the seventh-largest in the world based on applicable 2021 revenues. As of 2017, it is the biggest manufacturer in Britain. Its largest operations are in the United Kingdom and United States, where its BAE Systems Inc. subsidiary is one of the six largest suppliers to the US Department of Defense. Other major markets include Australia, Canada, India, Saudi Arabia, Turkey, Qatar, Oman and Sweden, where Saudi Arabia is regularly among its top three sources of revenue. The company was formed on 30 November 1999 by the £7.7 billion purchase of and merger with Marconi Electronic Systems (MES)the defence electronics and naval shipbuilding subsidiary of the General Electric Company plc (GEC)by British Aerospace, an aircraft, munitions and naval systems manufacturer.

BAE is the successor to various aircraft, shipbuilding, armoured vehicle, armaments and defence electronics companies. These include The Marconi Company, the first commercial company devoted to the development and use of radio; A.V. Roe and Company, one of the world's first aircraft companies; de Havilland, manufacturer of the Comet, the world's first commercial jet airliner; Hawker Siddeley, manufacturer of the Harrier, the world's first VTOL attack aircraft; British Aircraft Corporation, co-manufacturer of the Concorde supersonic transport; Supermarine, manufacturer of the Spitfire; Yarrow Shipbuilders, builder of the Royal Navy's first destroyers; Fairfield Shipbuilding and Engineering Company, builder of the world's first battlecruiser; and Vickers Shipbuilding and Engineering, builder of the Royal Navy's first submarines.

Since its formation in 1999, BAE has made a number of acquisitions, most notably of United Defense and Armor Holdings of the United States, and sold its shares in Airbus, Astrium, AMS and Atlas Elektronik. It is involved in several major defence projects, including the Lockheed Martin F-35 Lightning II, the Eurofighter Typhoon, the Astute-class submarine, and the Queen Elizabeth-class aircraft carriers. BAE Systems is listed on the London Stock Exchange's FTSE 100 Index.

British Aerospace plc (BAe) was a British aircraft, munitions and defence-systems manufacturer. Its head office was at Warwick House in the Farnborough Aerospace Centre in Farnborough, Hampshire. Formed in 1977, in 1999 it purchased Marconi Electronic Systems, the defence electronics and naval shipbuilding subsidiary of the General Electric Company plc, to form BAE Systems.