While espionage among nations is a long-standing practice, the emergence of the Internet has challenged the traditional legal framework and has resulted in the intensification of intelligence activities. With the emergence of cyber-espionage, agents may collect intelligence from within their own jurisdictions, with a great deal of secrecy and less risk. This book argues that — save some exceptions — this activity has been subject to normative avoidance, meaning that it is neither prohibited, nor authorized or permitted. States are aware of such status of law, and are not interested in any further regulation, leaving them free to pursue cyber-espionage themselves at the same time as they adopt measures to prevent and falling victim to it. This book resorts to a first-class sample of state practice and analyses several rules and treaties, and demonstrates that no specific customary law has emerged in the field.