Teen drivers ages 16 to 20 have the highest fatality rate of any age group in the United States. As a result, states have increasingly adopted laws to limit teen driving exposure, such as Graduated Driver Licensing (GDL) systems, which consist of three stages: a learner's permit allowing driving only under supervision; intermediate licensure allowing unsupervised driving with restrictions; and full licensure. The National Highway Traffic Safety Administration (NHTSA), within the Department of Transportation (DOT), supports state teen driver safety programs by researching teen driver safety issues, working to limit teens' access to alcohol, promoting seat belt use, and encouraging states to implement GDL systems. This requested report identifies (1) key GDL system requirements and the extent to which state programs include these requirements; and (2) challenges states face to improve teen driver safety and how states and NHTSA have addressed the challenges. Government Accountability Office (GAO) examined state GDL systems, visited six states, and interviewed federal and state traffic safety officials and other experts. GAO recommends that NHTSA conduct additional research on teen driver safety requirements such as entry age, passenger and nighttime driving restrictions, and driver education to help identify the optimum provisions of GDL systems. DOT officials reviewed a draft of this report and concurred with our recommendation. Appended are: (1) Objectives, Scope, and Methodology; (2) Recommended GDL Requirements; (3) Requirements of a GDL System and State Driver Safety Provisions; and (4) GAO Contact and Staff Acknowledgments. (Contains 5 tables, 3 figures, and 58 footnotes.)