Harold Adrian Russell "Kim" Philby was a British intelligence officer and a spy for the Soviet Union. In 1963, he was revealed to be a member of the Cambridge Five, a spy ring which had divulged British secrets to the Soviets during World War II and in the early stages of the Cold War. Of the five, Philby is believed to have been the most successful in providing secret information to the Soviets.

The Cambridge Five was a ring of spies in the United Kingdom that passed information to the Soviet Union during the Second World War and the Cold War and was active from the 1930s until at least the early 1950s. None of the known members was ever prosecuted for spying. The number and membership of the ring emerged slowly, from the 1950s onwards.