In the twenty-first century, cyberspace and the ‘real world’ have become inseparable. The stability and security of cyberspace therefore affect, in increasingly profound ways, the economies, international reputations, national security, military capabilities and global influence of states. In their short history, operations in cyberspace have already been used extensively by states and their non-state supporters for many purposes. They are an inevitable aspect of contemporary international affairs while carrying significant risk. In this Adelphi book Marcus Willett, a former deputy head of GCHQ, argues that there is no coherent or widely shared understanding of what cyber operations really are, how they are used and what they can do; or of their implications for strategic affairs and international law; or what their ‘responsible’ use really entails. The myths and misunderstandings that abound tend to dull the conceptual clarity needed by strategic policymakers and overseers, and they complicate the essential task in a liberal democracy of maintaining public consent for, and legitimisation of, the development and use of such capabilities. The book sheds light on these issues, exposing myths and clarifying misunderstandings.