|“||You can see everything your enemies can see!||”|
|“||Your spies have been sent out to watch enemy positions.||”|
|—Message once Spies is researched|
|“||[Player name] has sent out spies.||”|
|—Message when an ally has researched Spies|
|“||[Player name] has infiltrated your ranks with spies and now knows all that you do.||”|
|—Message when an enemy has researched Spies|
Spies is a technology in Age of Empires III that can be researched at the Capitol by European civilizations or the Town Center once the Imperial Age is reached by Native American and Asian civilizations. Once researched, it allows the player to see their enemies' Line of Sight.
As with its Age of Empires II version and similar to Blockade, Spies have a variable cost that depends on how many units the enemy has, but with no minimum or maximum limit.
In a team game, only the players who researched the upgrade can see the enemy's line of sight, it is not shared with teammates.
Except if the starting age is set to Imperial/Post-Imperial Age, Spies is unavailable in Deathmatch as players cannot advance into the Imperial Age.