[en] The regulatory authority and oversight of radioactive materials in the U.S. has continued to expand due to the potential for malevolent use, and thus more focused attention on security and control of these materials following the events of September 11, 2001. The Category 1 and 2 quantities of radioactive sources listed in the International Atomic Energy Agency (IAEA) Code of Conduct on the Safety and Security of Radioactive Sources are considered the most risk significant for the United States and have been the focus of Federal and State efforts to place tighter controls for security - especially during times of elevated risk of loss, theft, and diversion, such as during transport. Transportation security of these risk-significant materials is especially complex due to the number of U.S. Government agencies that have various roles and authority for implementation and enforcement of applicable regulations. This paper will focus on the significant progress that has been made with regard to interagency coordination and communication on the secure transportation of radioactive materials within the U.S. or across U.S. borders and with the adoption of new regulations (10 CFR Part 37) on March 19, 2013. These regulations established security requirements for the use and transport of these risk-significant radioactive materials, as well as for shipments of small amounts of irradiated reactor fuel. Elements of these regulations address areas such as preplanning and coordinating shipments; advance notification of shipment details to the NRC and U.S. States through which the shipment will pass; control and monitoring of shipments that are underway; trustworthiness and reliability of personnel; and information security considerations. This paper will also address NRC's current efforts on performing a retrospective program review of 10 CFR Part 37 to ensure an effective and efficient framework for the security, including transportation security, of radioactive material. The wide expanse of this assessment, which includes an internal and external assessment, analysis of international regulations and standards, and stakeholder outreach will inform decisions made regarding the various provisions addressed by the new regulations, including the area of physical protection in transit. (author)