G4S plc (formerly Group 4 Securicor) is a global security services company headquartered in Crawley, United Kingdom. It is the world's largest security company measured by revenues and has operations in more than 125 countries. With over 625,000 employees it is the world's second-largest private sector employer. Its primary listing is on the London Stock Exchange and it is a constituent of the FTSE 100 Index. It has a secondary listing on the Copenhagen Stock Exchange.