Over the course of the 20th century, the secretive government agencies and the spies who ran them would complete the transformation of espionage from an amateur activity to a full-time profession. Nations have come to rely on spies for protection from terrorists, other spies, and attacks by enemies. Secrecy keeps their activities out of sight until a rogue agent is caught using espionage for treasonous or greedy ends, or when their efforts to protect us fail. But as spy-tools grow more and more sophisticated, one thing is certain: espionage is here to stay.