BAE Systems plc (BAE) is a British defence and aerospace company headquartered at Farnborough, UK, which has worldwide interests, particularly in North America through its subsidiary BAE Systems Inc. BAE is the world's third largest defence contractor and the largest in Europe. BAE was formed on 30 November 1999 by the £7.7 billion merger of two British companies: Marconi Electronic Systems (MES), the defence electronics and naval shipbuilding subsidiary of The General Electric Company plc (GEC) and aircraft, munitions and naval systems manufacturer British Aerospace (BAe).

BAE is the successor to many iconic aircraft and defence electronics companies, including 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 world's first commercial jet airliner; British Aircraft Corporation, co-manufacturer of the Concorde supersonic transport; and Supermarine, the manufacturer of the renowned Spitfire. It has increasingly disengaged from its businesses in continental Europe in favour of investing in the United States. Since its formation it has sold its shares of Airbus, EADS Astrium, AMS and Atlas Elektronik.

BAE Systems is involved in several major defence projects, including the F-35 Lightning II, the Eurofighter Typhoon and the Royal Navy Queen Elizabeth class aircraft carriers. The company has been the subject of criticism, both general opposition to the arms trade and also specific allegations of unethical and corrupt practices, including the Al Yamamah contracts with Saudi Arabia that have earned BAE and its predecessor £43 billion in twenty years.