Their most recent heyday was the Cold War. But defectors have been around as long as states have been fighting each other. Think, for example, Benedict Arnold or Rudolf Hess. Now this eclectic company has been joined by Moussa Koussa, until lately foreign minister of Libya, and now a guest at a "secret undisclosed location" of Her Majesty's Government.
His immediate debriefing will surely focus on the power structure around Colonel Gaddafi, as the British seek "live" information about Libya's government, intelligence and military.
But why has he changed sides? Defection is a treacherous business. People cross sides for many reasons. Some do so because they truly believe their country is in the wrong, and that to continue to serve it is immoral. The very bravest (or reckless) of these such as Colonel Oleg Penkovsky, perhaps the most important of all Cold War spies, do so without leaving their country, and pay for it with their lives. Others defect from spite, after a career setback or spurred by a feeling of being overlooked. For some, it is the only way to save their skins; others are driven by a love affair. A few, like Hess, defect out of delusion.
For the recipient country, most defectors are good news. But some are not. One of the most famous post-Cold War specimens is Rafid Ahmed Alwan al-Janabi, the Iraqi scientist known as "Curveball". His false information about Saddam Hussein's biological weapons programme was swallowed hook, line and sinker by anti-Saddam hawks in Washington as justification for the 2003 invasion.
Nor are all defections forever. An example of immense consequence was that of a former US Marine private named Lee Harvey Oswald, who in 1959 renounced his citizenship and settled in the Soviet Union, then changed his mind and returned home in May 1962.
And the Yurchenko affair is still among the most puzzling. When he approached the Americans in Rome in August 1985, Vitaly Yurchenko was touted as one of the most important KGB officers to cross to the West. Just three months later, he slipped his CIA handlers and re-defected to the Soviet Union. For this successful "infiltration operation", he was reportedly awarded the Order of the Red Star. It is unlikely any such honour would await Mr Koussa in Tripoli, if he returned home.