A successful terrorist attack on a facility containing nuclear weapons could have devastating consequences. GAO was asked to compare the Department of Defense's (DOD) and Department of Energy's (DOE) efforts to protect the nation's nuclear weapons where they are stored, maintained, or transported. This report (1) compares the nuclear weapons security policies and procedures at DOD and DOE, and the extent to which cost-benefit analyses are required; (2) compares DOD and DOE efforts to assess threats to nuclear weapons; and (3) identifies total current and projected funding requirements for securing nuclear weapons, including military construction costs. GAO analyzed DOD and DOE nuclear weapons security policies and procedures; visited sites that store, maintain, or transport nuclear weapons; and analyzed funding data for fiscal years 2006 through 2013. This report is an unclassified version of a classified report issued in May 2009. GAO recommends that the Secretary of Defense improve DOD's process for evaluating and selecting among alternative security measures, improve installation commanders' ability to assess threats, and improve visibility and accountability over Air Force nuclear weapons security funding. DOD partially agreed with the recommendations in this report, noting several actions in process or needed to address the recommendations.