The Department of Defense (DOD) offers education and training to foreign military personnel--known as international military students--at United States (U.S.) DOD sites. After a student killed 3 people at a base in Florida in 2019, questions were raised about personnel safety at DOD sites hosting students for training. The William M. (Mac) Thornberry National Defense Authorization Act for Fiscal Year 2021 includes a provision for the Government Accountability Office (GAO) to review DOD's implementation of security vetting for international military students and accompanying family members. This report assesses the extent to which: (1) DOD has developed vetting procedures for international military students and addressed any implementation challenges; and (2) State and DOD have shared information related to students' training at DOD sites in the U.S. GAO reviewed documentation, analyzed vetting result data for October 1, 2019, through March 31, 2023, and interviewed DOD and State officials and stakeholders from six unified combatant commands and three U.S. embassies. Some stakeholders found opportunities for improvement, and GAO is making four recommendations, including for DOD to assess vetting implementation and take action to mitigate any factors affecting implementation as identified in its assessment, and for DOD and State to clarify roles and responsibilities for sharing information.