The Department of Homeland Security's (DHS) Domestic Nuclear Detection Office (DNDO) is testing new advanced spectroscopic portal (ASP) radiation detection monitors. DNDO expects ASPs to reduce both the risk of missed threats and the rate of innocent alarms, which DNDO considers to be key limitations of radiation detection equipment currently used by Customs and Border Protection (CBP) at U.S. ports of entry. Congress has required that the Secretary of DHS certify that ASPs provide a significant increase in operational effectiveness before obligating funds for full-scale procurement. GAO was asked to review (1) the degree to which DHS's criteria for a significant increase in operational effectiveness address the limitations of existing radiation detection equipment, (2) the rigor of ASP testing and preliminary test results, and (3) the ASP test schedule. GAO reviewed the DHS criteria, analyzed test plans, and interviewed DHS officials. GAO recommends that DHS assess ASPs against the full potential of current equipment and revise the program schedule to allow time to conduct computer simulations of ASPs' capabilities and to uncover and resolve problems with ASPs before full-scale deployment.