In July 2003, Finmeccanica and BAE Systems signed a joint venture agreement with the intention merging their avionics, C4ISR and communications businesses. A revised agreement was signed in 2005. Finmeccanica and BAE announced the intention to dissolve their partnership in the Alenia Marconi Systems (AMS) joint venture with AMS' UK and Italian operations to be taken over by the respective partners.

The whole of the avionics business of Finmeccanica and the majority of BAE Systems Avionics were brought together to form the new SELEX Sensors and Airborne Systems (SELEX S&AS) joint venture held by shares of 75% and 25% respectively, with the option for Finmeccanica to later purchase the remaining 25%.

BAE Systems sold its 25% share of SELEX S&AS to Finmeccanica for €400 million in 2007.

From 2008, SELEX S&AS Limited and its Italian counterpart Galileo Avionica S.p.A. began to operate under a common brand name, SELEX Galileo. In January 2010, the operating companies’ legal names were aligned to the brand, becoming SELEX Galileo Ltd and SELEX Galileo S.p.A. respectively. Selex ES was a subsidiary of Finmeccanica S.p.A. (today Leonardo), active in the electronics and information technology business, based in Italy and the UK, and formed in January 2013. This followed Finmeccanica's decision to combine its existing SELEX Galileo, SELEX Elsag and SELEX Sistemi Integrati businesses. 

From 1 January 2016, the activities of Selex ES merged into Leonardo-Finmeccanica’s Electronics, Defence and Security Systems Sector.