A well-informed defector is the most dangerous counter-intelligence commodity because it takes a spy to catch a spy. Very occasionally, an agent, especially a mole or an intelligence professional, will make a mistake and incriminate themselves, but usually it is a denunciation, a tip, or a vague clue from a defector that will provide the vital information required to expose the source of a leak. Relying on recently-declassified intelligence files and interviews with defectors, their handlers, their families, and their victims, Nigel West has analysed nine examples of wartime and postwar defections to shed new light on the phenomenon.