Cargo containers could be used to transport unlawful cargo, including weapons of mass destruction, illicit arms, stowaways, and illegal narcotics into the United States. Within the Department of Homeland Security (DHS), U.S. Customs and Border Protection (CBP) is responsible for container security. To enhance container security, CBP has partnered with DHS's Science and Technology (S & T) Directorate to develop performance standards -- requirements that must be met by products to ensure they will function as intended -- for container security technologies. After successful completion of testing, S & T plans to deliver performance standards to DHS's Office of Policy Development and CBP. As requested, this report addresses (1) the extent to which DHS has made progress in conducting research and development and defining performance standards for the technologies, and (2) the remaining steps and challenges, if any, DHS could face in implementing the technologies. GAO, among other things, reviewed master test plans for S & T's four ongoing container security technology projects, and interviewed DHS officials. GAO recommends that DHS test and evaluate the container security technologies consistent with all the operational scenarios DHS identified for potential implementation. DHS concurred with our recommendation.