The place of espionage under international law is an uneasy one. Many look to state practice, and conclude that acts of espionage must surely be legal because so many states spy on one another.1 Others say that sending spies in to the territory of another state breaches the norm of territorial and sovereign integrity, and therefore acts of espionage should rightly be considered illegal pursuant to the relevant legal rules.2 The difficulty in assessing legality is amplified when analyzing cyber intrusions, because they do not offend the principle of territorial integrity in the same way that sending an actual state agent to gather human intelligence (colloquially referred to as “HUMINT”) would. This paper is an attempt to address these legal ambiguities.