Building NMMSS Resilience in Response to Global Nuclear Proliferation Threats This paper describes the role of the United States Nuclear Materials Management and Safeguards System (NMMSS) in its role to provide mission-critical data analytics, products and services that inform, and advance U.S. government policies and responses related to global nuclear proliferation threats. The sources of NMMSS data are from domestic facilities and foreign nations or organizations and are submitted under contractual requirements, government rules and regulations, and international agreements. NMMSS data have supported U.S. global nonproliferation initiatives since the 1970s. The interdependencies between domestic and international nuclear materials data uniquely enable NMMSS to provide services for other organizations such as the International Atomic Energy Agency (IAEA), National Nuclear Security Administration (NNSA) Office of Defense Nuclear Nonproliferation, Nuclear Regulatory Commission (NRC), Department of State, and the U.S. Congress. Consequently, building resilience, including adapting and managing change effectively and creatively, is of paramount importance to the NMMSS program. This paper presents lessons learned, noteworthy practices, and recommendations of potential benefit for other IAEA member states using similar materials control and accounting systems.