While espionage between states is a practice dating back centuries, the emergence of the internet revolutionised the types and scale of intelligence activities, creating drastic new challenges for the traditional legal frameworks governing them. This book argues that cyber-espionage has come to have an uneasy status in law: it is not prohibited, because spying does not result in an internationally wrongful act, but neither is it authorised or permitted, because states are free to resist foreign cyber-espionage activities. Rather than seeking further regulation, however, governments have...