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 criminals smuggling drugs or counterfeit goods, to unauthorized 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 organized around risk management, a process that involves risk assessment and the allocation of resources based on a cost-benefit analysis. This report focuses on the first part of this process by identifying border threats and describing a framework for understanding risks at U.S. borders. 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. Members of Congress may wish to use similar models to evaluate the costs and benefits of potential border security policies and to allocate border enforcement resources. This report discusses some of the issues involved in modeling border-related threats. Understanding border risks begins with identifying key threats. At their roots, border-related threats are closely linked to the flow of people (travelers) and goods (cargo) from one country to another. Any smuggled item or individual hidden among the legitimate flows potentially constitutes a threat to U.S. security or interests. The intentions and actions of unauthorized travelers separate them into different threat categories, including terrorists, transnational criminals, and other illegal migrants.