Over the past two decades, the notion of ‘health security’ has emerged as a central tenet of European Union (EU) public health policy. This PhD thesis examines the rise and implications of health security cooperation, associated with an imperative to fight ‘bioterrorist attacks’, pandemics and other natural or man-made events. The study is composed of an introductory chapter as well as five related but self-contained papers, based on participant observation and 52 in-depth interviews at the European Commission as well as the European Centre for Disease Prevention and Control (ECDC). More specifically, the thesis as a whole explores how security perspectives mattered in different ways for the rise and implications of health security cooperation in the EU. Unlike previous studies which have tended to focus on normative aspects and overarching global dynamics, the thesis examines drivers, contradictions and tensions in a particular, highly institutionalized context. In order to answer a set of empirically motivated questions, the papers draw on various understandings of securitization in critical security studies. The over-all findings cast light on the emergence of a new way of understanding health problems as rapidly emerging, and often external, ‘cross-border threats to health’. The latter may include major infectious disease outbreaks, but also deliberate or accidental release of chemical or biological substances, natural disasters or any other unknown event assumed to threaten not only public health but society as a whole. In the search for potential crises, these are to be rapidly detected and contained rather than prevented in line with traditional public health policy. Partly arising from political speech acts after September 11 as well as bureaucratic practices carving out a role for the EU in public health, these new priorities have also been shaped by EU-specific digital surveillance tools, information sharing platforms and methodologies for managing risk. The findings also point to forms of reflexivity and instances of contestation within the EU institutions themselves, especially in relation to migrant health. As a whole, the thesis thus contributes empirically to a better understanding of how both health and security have come to be pursued within the EU institutions. Theoretically it highlights how approaches to securitization, drawn from partially different scholarly traditions, can be employed as empirically sensitive analytical tools and thereby add to a better understanding of the full prism of securitization processes.