This volume explores the important contribution of technical experts to shaping states’ perceptions of what international rules apply to their conduct in cyberspace, and how those rules regulate various cyber operations. Drawing on 67 interviews of diplomats, lawyers, and technical experts, along with nine months of immersive observation of international cybersecurity negotiations, this book offers a sweeping assessment of how technical expertise has influenced legal discourse on key issues such as cyber countermeasures, election hacking, sovereignty, due diligence, and protection of core internet infrastructure. The book demonstrates how the articulation of informal “cyber norms” became intertwined with the application of existing international law at the UN and multistakeholder forums over two decades. Technical experts, who were active participants in norms development processes, therefore became direct interlocutors in the creation and interpretation of international cybersecurity law – a phenomenon this book terms “creeping expertise”.