Unmanned aircraft systems (UAS), commonly called drones, have proliferated rapidly and are available to nation states and to nonstate actors and individuals. These systems could provide U.S. adversaries with a low-cost means of conducting intelligence, surveillance, and reconnaissance missions against—or attacking—U.S. forces. Furthermore, many smaller UASs cannot be detected by traditional air defense systems due to their size, construction material, and flight altitude. As a result, in FY2021, the Department of Defense (DOD) plans to spend at least $404 million on counter-UAS (C-UAS) research and development and at least $83 million on C-UAS procurement. As DOD continues to develop, procure, and deploy these systems, congressional oversight of their use may increase, and Congress may have to make decisions about future authorizations, appropriations, and other legislative actions.