This book provides a pathbreaking attempt both to define the important legal questions related to the growing use of "big data" in extraterritorial military operations, and to begin to provide some answers. Big data, meaning the troves of data generated by new information technologies and the advanced analytics used to process that data, is radically reshaping the modern battlefield. Like many new military technologies and capabilities, the myriad uses of big data present broad questions about how to translate existing rules and principles embedded in multiple bodies of law to these new contexts, both within armed conflict, as part of adversarial activities below the armed conflict threshold, and in a range of related operations that increasingly use, deploy, and target such data. These questions extend beyond the role of big data within weapons systems and other military capabilities to questions about the nature of civilian harm, scope of individual rights, atrocity investigation, and humanitarian relief. The chapters in this book comprise the first initiative to grapple with a wide swath of these questions including whether, and how, jus ad bellum, international humanitarian law, international human rights law, and international criminal law might apply to operations involving big data. At the same time, because big data is so transformative, the uses of such data provoke deeper questions about the law itself, exposing gaps and interpretive ambiguities in existing legal frameworks that generate critiques of those frameworks as inadequate. Accordingly, while big data holds enormous promise, it also has the potential to disrupt modern warfare and the rule of law itself. This book confronts these issues directly, offers a range of approaches, and suggests an initial roadmap for scholars and practitioners alike.