Research suggests that youth with Autism Spectrum Disorder (ASD) are less likely than youth with other disabilities to be successful in transitioning to work and postsecondary education and therefore, they may face a lifetime of reliance on public assistance. The U.S. Government Accountability Office (GAO) was asked to examine services provided under the Individuals with Disabilities Education Act (IDEA) to assist youth with ASD in transitioning to adulthood. For this report, GAO examined (1) services and supports provided to assist youth with ASD in transitioning to adulthood, (2) key challenges in successfully transitioning, and (3) the extent to which federal agencies have collaborated to assist in the transition. GAO reviewed relevant federal laws and regulations and conducted a nationally generalizable survey of 588 school districts to gather information on services provided in school year 2015-16. GAO also interviewed federal officials and state and local stakeholders in three states selected to highlight a mix of localities with and without initiatives serving this population, urbanicity, and geographic dispersion, and GAO evaluated federal collaborative efforts against leading practices. GAO is making three recommendations to federal agencies, including one to Education to examine the merits and implications of amending IDEA to require earlier transition planning and one to the Department of Health and Human Services' (HHS) to enhance collaboration with non-Interagency Autism Coordinating Committee (IACC) member agencies. Education neither agreed nor disagreed with the IDEA recommendation. The agencies generally agreed with or did not comment on other recommendations.