BAE Systems plc is a British multinational defence, security, and aerospace company. Its headquarters are in London in the United Kingdom and it has operations worldwide. It is among the world's largest defence companies; it was ranked as the third-largest based on applicable 2015 revenues. 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, India, and Saudi Arabia, which account for about 20% of BAE's overall sales. It is the biggest manufacturer in Britain. The company 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 British Aerospace (BAe) – an aircraft, munitions and naval systems manufacturer.
BAE Systems 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 and is a constituent of the FTSE 100 Index.