In politics, a defector is a person who gives up allegiance to one state in exchange for allegiance to another, changing sides in a way which is considered illegitimate by the first state. More broadly, defection involves abandoning a person, cause, or doctrine to which one is bound by some tie, as of allegiance or duty.

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.