Sid Meier's Civilization V: Gods & Kings/Espionage
Espionage is a feature that is implemented in the Gods and Kings expansion. As one might guess, it lets you obtain useful knowledge about other civilizations, though espionage also has other uses. The Espionage system consists of multiple spies. A civilization starts off with 1 spy at the Renaissance Era and gains a spy when entering every new technological era. The exception to this rule is England, which gets 2 spies at the start of the Renaissance Era.
Unlike in past Civilization games, spies are not units visible on the map. One must order them to move to cities using the Espionage overview, and the spy will take 1 turn to move there. Once a spy reaches a city, they will perform their tasks automatically. You cannot have more than 1 spy in any city. A spy has three ranks: Recruit, Agent and Special Agent, and spies can level up to a new rank once enough experience is gained. However, it is possible for a spy to be killed if found out, getting replaced by a new, more inexperienced spy.
Spying major civilizations
You can send a spy to one of the cities of a normal civilization, which will give you sight of the city and all adjacent tiles around it as well as letting you access the city screen as if it was your own. While you can't influence what the city does, you can check building it has, yields from its tiles and what it is currently producing.
A spy can do two things while in an opponent's city: steal technologies are gather intrigue. As soon as a spy arrives at an enemy city, they will establish surveillance of that city, letting you know of its potential. If you're so scientifically advanced that the owner of the city has no technologies you can steal, all of their cities will basically grant zero potential. However, if the owner of the city has a technology you don't, your spy can steal it, depending on the city's potential. A city with a high potential rating means you will be able to steal technologies faster. Be aware that a spy may be detected and even killed if the city has counter-intelligence spies or buildings.
No matter whether the city has any technologies to steal or not, you can still leave a spy there to intercept intrigue. For example, your spy may uncover a plot by a civilization to attack another. You can share intrigue with the civ in question to gain a diplomatic bonus with that civilization. Other civilizations can also share intrigue with you, but they can't obtain intrigue about any human players. Intrigue about enemy movements will seldom come true, simply because the civilization in question might suddenly change its mind.