Harold Adrian Russell "Kim" Philby (1 January 1912 – 11 May 1988) was a British intelligence officer and a double agent 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 most successful in providing secret information to the Soviets.Born in British India, Philby was educated at Westminster School and Trinity College, Cambridge. He was recruited by Soviet intelligence in 1934. After leaving Cambridge, Philby worked as a journalist, covering the Spanish Civil War and the Battle of France. In 1940 he began working for the United Kingdom's Secret Intelligence Service (SIS or MI6). By the end of the Second World War he had become a high-ranking member. In 1949 Philby was appointed first secretary to the British Embassy in Washington and served as chief British liaison with American intelligence agencies. During his career as an intelligence officer, he passed large amounts of intelligence to the Soviet Union, including a plot to subvert the communist regime of Albania.

Philby was also responsible for tipping off two other spies under suspicion of espionage, Donald Maclean and Guy Burgess, both of whom subsequently fled to Moscow in May 1951. The defections of Maclean and Burgess cast suspicion over Philby, resulting in his resignation from MI6 in July 1951. He was publicly exonerated in 1955, after which he resumed his career as both a journalist and a spy for SIS in Beirut, Lebanon. In January 1963, having finally been unmasked as a Soviet agent, Philby defected to Moscow, where he lived until his death in 1988.