Next to sea, land, air and space, ‘cyber space’ appears to be the fifth operational domain for the military. This manmade and virtual sphere brings along opportunities and threats. In this book, academics of the Netherlands Defence Academy as well as specialists and military professionals from other institutions analyse developments in cyber security. The authors use various fields of expertise, enabling them to apply a truly multi-disciplinary approach. Cyber threats affect states’ vital interests. By consequence, responses are required that need to come from public authorities as well as organizations on the market and in civil society. Additionally, this is not a national state’s affair; cyber threats are an international concern.This collection of essays questions the premises and scenarios on which policy responses are based. Are these scenarios describing real or imaginary threats? A number of contributions specifically deal with this issue. This volume also covers the process of ‘securitization’, the legal aspects of cyber operations, the use of social media in military operations as well as an economic analysis of the costs of cyber dangers. Furthermore, the authors analyse organisational responses to cyber threats. Resilience is the central term in this respect, both at the individual and the organisational level of military HQs and strategic network management. An analysis based on operations research and game theory illustrates the vulnerabilities of digital networks. No matter how dangerous cyber threats may look at face value, one should not overdramatise the dangers, as the last chapters seem to imply.In all contributions, the authors have attempted to combine academic theory with practical views and suggestions. This makes this book valuable not only for people from academic life, but particularly also for people who struggle with the intricacies and worries of cyber threats in their everyday (professional) life.