The Cambridge Five consisted of Kim Philby, Donald Maclean, Guy Burgess, Anthony Blunt, and John Cairncross, all Cambridge graduates, who made their careers in various British government agencies including the Secret Intelligence Service.
They were recruited to work for Stalin's NKVD (the precursor to the KGB) while students at Cambridge in the 1930s by Arnold Deutsch, a Russian talent spotter. Many British students at that time saw the rise of Fascism as dangerous and felt that only the Soviet Union was powerful enough to stand up to the threat.
This belief made it relatively easy for the USSR's secret service to recruit select high-flying students from prestigious universities in the hope that they would rise to positions of influence in their future careers. In the case of the Cambridge Five the investment paid off - during their careers they passed countless numbers of classified documents to Soviet authorities.
In the 1950s Philby, Maclean and Burgess were ...