With growing dependency on digital infrastructure, vulnerability to cyber disaster becomes a defining context for social life. Within the last two years Wannacry led to the cancellation of thousands of NHS appointments, NotPetya brought Maersk's global shipping operations to a halt, the Equifax hack compromised the details of 140 million people, and TSB's outage left thousands of customers defrauded. Behind these failures-to patch systems, to secure networks, to implement good governance-is a problem of scales: the smallest "weak link" can end up compromising the security of the whole system. Yet because complete security is unattainable in practice, living well with infrastructures has become a question of trust. It is the premise of this fellowship that trust is not a "user's problem". Behind the services and utilities that we rely on in daily life, we can find an array of professional cyber security practices aiming to win and maintain trust, to question it and manage it across scales. How they go about doing that, their successes and failures, is the subject of this study. The ambitious anthropological study of cyber security at the heart of this fellowship will be undertaken in collaboration with the UK's National Cyber Security Centre (NCSC). A broad programme of ethnographic research will focus on long term participant observation of governance processes and knowledge practices within Critical National Infrastructure (CNI) organisations. Three trajectories of investigation comprise the core of the fellowship: In years 1-2, with an ethnographic study of the implementation of the Network and Information Systems directive in 2 CNI locations, it asks: how does cyber security policy "scale" best practice into diverse real-world contexts? In years 3-4, with an ethnographic analysis of how trust is built through the "rituals" of corporate governance in 3 CNI locations, it asks: how do IT practitioners "scale up" local forms of trust to create "high level" holistic representations with which approval can be given, and responsibility taken? In years 5-7, together with a postdoc, the fellow will conduct an examination of the impact of new technologies of automation and AI on cyber security practice, it asks: how do new technologies reconfigure trust? Traditionally led by engineers, cyber security has a legacy of treating people or users simplistically: as problems, or attack vectors. Interdisciplinary approaches have had steady success over recent years in developing more nuances approaches. This fellowship advances the state of the art in interdisciplinary cyber security research with an anthropological style of empirically grounded critical conceptual analysis of professional practices involved in making and managing trust across scales. In doing so it will also make important contributions to several fields of research in the social sciences: the anthropology of governance and accountability, the sociology of trust, and interdisciplinary studies of the digital infrastructures that underlie contemporary social societies. A comprehensive impact programme will ensure that the study stays aligned with policymakers' priorities, and contributes to cyber security policy and practice across industry and government. Academic audiences will be reached through presentation at leading conferences and an ambitious publication strategy targeting high impact journals, and an academic monograph, aiming to be a definitive anthropological account of cyber security. The fellow's professional background managing digital and IT projects are indispensable to this research, as is his research experience in the ethnography of computational science. An extensive training and discipline hopping programme will make the fellow a research leader, standing between academic fields, industry, and policy, poised to produce the engaged interdisciplinary research needed to tackle the Grand Challenges of the UK's Industrial Strateg