The Plessey Company plc was a British-based international electronics, defence and telecommunications company. It originated in 1917, growing and diversifying into electronics. It expanded after the Second World War by acquisition of companies and formed overseas companies.
It was listed on the London Stock Exchange and was a constituent of the FTSE 100 Index. In 1989, it was taken over by a consortium formed by GEC and Siemens which split the assets of the Plessey group.
The majority of Plessey's defence assets were amalgamated into BAE Systems in 1999 when BAe merged with the defence arm of GEC, Marconi Electronic Systems (MES). A small portion of the defence market, mostly embedded electronic systems and circuitcards remained with GE, formerly GE Fanuc and now GE Intelligent Platforms (GE-IP) based in Towcester. The bulk of Plessey's telecommunications assets were acquired by Ericsson through its 2005 acquisition of Marconi Corporation plc, a successor company of GEC. The remainder of the communication assets went to Telent plc.