The United States confronts a wide array of threats at U.S. borders, ranging from terrorists who may have weapons of mass destruction, to transnational criminal smuggling drugs or counterfeit goods, to unauthorised migrants intending to live and work in the United States. Given this diversity of threats, how may Congress and the Department of Homeland Security (DHS) set border security priorities and allocate scarce enforcement resources? In general, DHS's answer to this question is organised around risk management, a process that involves risk assessment and the allocation of resources based on a cost-benefit analysis. DHS employs models to classify threats as relatively high- or low-risk for certain planning and budgeting exercises and to implement certain border security programs. This book examines risk management as it relates to national security threats with a focus on border security; chemical, biological, radiological, and nuclear risk assessments; and the publishing of scientific papers with potential security risks.