This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book aims is to present examinations of cyber warfare from a variety of disciplinary perspectives. It is focused on acts of cyber aggression as between countries, rather than as instigated by individuals or (solely by) non-state groups. The book argues that cyber warfare is an emerging phenomenon in international relations, with state-orchestrated, or apparently state-orchestrated, computer network attacks against other states occurring with increasing frequency and scale. Yet a crucial impediment to the understanding of cyber warfare and its actual and potential implications is that there is a general lack of understanding of what the phenomenon entails in practice. If cyber warfare is to be properly understood and its injurious consequences limited, there is a need for deep interdisciplinary analysis.