Uncrewed aircraft systems, also known as "drones," are used to take photos, deliver packages, monitor crops, and more. However, drones can pose significant safety and security risks—for example, if they enter airspace near airports or if they're used for illegal activities such as drug smuggling. To reduce these risks, counter-drone technology can detect unauthorized or unsafe drones and, when needed, jam, capture, or disable them. We examined how the counter-drone systems work, the maturity of these technologies, and some of the opportunities, challenges, and policy questions posed by the use of these systems.