Advances in science and technology, the rise of globalization, the emergence of new diseases, and the changing nature of conflict have increased the risks posed by naturally occurring and man-made biological threats. A growing acceptance of a broader definition of security since the end of the Cold War has facilitated the rise of biosecurity issues on the international security agenda. Developing strategies to counter biological threats is complicated by the lack of agreement on the definition of biosecurity, the diverse range of biological threats, and competing perspectives on the most pressing biological threats. A comprehensive definition of biosecurity that encompasses naturally occurring, accidental, and deliberate disease outbreaks can help to further research, analysis, and policymaking. Operationalizing this broad conception of biosecurity requires a taxonomy of biological threats based on a levels-of-analysis approach that identifies which types of actors are potential sources of biological threats and the groups most at risk from these threats. A biosecurity taxonomy can provide a common framework for the multidisciplinary research and analysis necessary to assess and manage these risks. It also has implications for how to prevent and respond to biological threats, as well as for the future of biosecurity research.