In the spy game, listening, learning and gaining the ability to eavesdrop requires ingenuity.
For some covert missions, assassination is the goal, and lethal toxins are deployed by spy agencies to kill perceived foes.
As long as there has been espionage, sex and the promise of it have been powerful tools of persuasion and blackmail.
Methods for capturing sensitive data have grown more advanced as devices have become exponentially smaller.
Intelligence officers acquire the information collected by sources, from old-school "dead drops" to high-tech digital transfers.
Special operations require skillful people, but some of the most famous missions in modern history relied on elite technology.
Motivations leading people to spy against their own nations tend to fall into one of four categories, as notorious cases demonstrate.