Sandia National Laboratories has been supporting the Domestic Nuclear Detection Office (DNDO) of the Department of Homeland Security (DHS) since DNDO’s inception in 2005. Sandia provides some level of support for each of the seven main DNDO directorates, from basic research and development in radiation detector materials to 24/7 technical reachback response. Our contributions range from fundamental to applied research and development, to systems analysis and test and evaluation, to day-to-day operational support and forensics and attribution. Sandia applies extensive knowledge and experience and a unique array of laboratories and equipment to contribute to solutions for improved defense against the domestic nuclear threat. For instance, Sandia contributes to forensic activities by performing post-event sampling and analysis of trace elements in nuclear materials, which can provide a fingerprint to identify the material’s source. Certain identification of the source of a nuclear weapon or RDD material allows appropriate response, which in turn can help significantly deter future attacks.