In the U.S. Department of Energy, the National Nuclear Security Administration -- a semi-autonomous agency -- is responsible for securing fully and partially assembled nuclear weapons and significant quantities of special nuclear material at facilities around the country. The U.S. Congress directed the Administration to ask the National Research Council for advice on augmenting its security approach, particularly on the applicability of quantitative and other risk-based approaches for securing its facilities. The National Research Council committee concluded that balancing cost, security, and operations at facilities in the nuclear weapons complex cannot be accomplished by assessing security risks more quantitatively or more precisely. However, using qualitative, structured thinking processes and techniques to characterize security risk could improve the Administration's understanding of security vulnerabilities and guide more effective resource allocation. Specifically, the committee identified three key shortcomings in NNSA's current security approach that could -- at least in part -- be addressed by the adoption of such processes and techniques. A "total systems approach" could better serve the interests of security by better characterizing the interactions and dependencies of security countermeasures at Department of Energy facilities.