There have been a few ways the Civ games have handled spies, but I like the idea of spies being units that you can control. The premises is simple, you give your unit the Spy Profession and send him out on missions such as: Sabotage Gather information Incite revolts Destroy Improvements Counter Espionage Infiltration Falsify Information Infiltration Here is the cool one. You can send your experienced spies on infiltration missions. Basically your spy will inter the que for the enemies Immigration and once the spy has successfully infiltrated the enemy you can perform the missions more effectively and less chance to get caught. The enemy will use your unit as if it was its own. Special missions are allowed to infiltrators such as a research bonus to techs you are researching that the enemy already has. Double Agents Here is the real killer, if your infiltrated unit is given the Spy Profession by the enemy you now have a double agent. Double agents receive intel on the where abouts of other spies, and receive a huge bonus to counter espionage, among other benefits. Very Experienced Double Agents can even be sent on infiltration missions again and become a Triple Agent, gaining even more bonuses. After each mission, successful or not, the Spy attempts to go into hiding and become unavailable for a few turns. Infiltrators and Double Agents attempt to return to their jobs as normal. Depending on a spy's skill the longer he works the more suspicion he builds up, if his suspicion gets to high his covered could get blown and he would need to escape. Once your cover is blown for that Civ you can no longer Infiltrate that Civ. (In a world History Mod certain techs, like Plastic Surgery, can change this) Depending on how well Infiltrators and Double Agents perform missions, they can leave a trail of evidence that increases the Agents suspicion levels. Victims of Espionage can thus receive messages, little hints, that there may be a spy among your populace. Falsify Information This special mission can be performed by your spies. It attempts to draw the heat away from a real spy by planting falsehoods, or a false trail of evidence, that can make a player think there is a spy somewhere that there is not. It can lower the suspicion level of real spies. In the end, you just have no idea who you can really trust. Pray my child, pray!