Beginning in 2011, the Department of Education (Education) used its statutory authority to invite states to apply for waivers from certain provisions in the Elementary and Secondary Education Act (ESEA) through its Flexibility initiative. To receive Flexibility waivers, states had to agree to meet other requirements related to college- and career-ready expectations, school accountability and support, and effective instruction. Education approved Flexibility waivers for 43 states. In December 2015, Congress reauthorized the ESEA, which modified Education's waiver authority. The Government Accountability Office (GAO) was asked to review Education's Flexibility initiative. GAO examined the extent to which Education assessed states' ability to fully implement their Flexibility waivers and the process it used to oversee the waivers. GAO reviewed relevant federal laws, guidance, and key documents related to the Flexibility initiative, such as monitoring reports; and interviewed Education officials. GAO reviewed Education's documents and identified states facing multiple challenges in implementing their waivers. GAO also interviewed officials in five states, selected to reflect a range of challenges states faced in implementing the waivers. GAO recommends that Education evaluate its Flexibility initiative oversight process to identify lessons learned and incorporate any applicable lessons into its plans for overseeing state implementation of the new law. Education generally agreed and outlined steps to address the recommendation. Appended are: (1) Key Benefits and Challenges of Flexibility Waivers, According to Officials in Selected States; (2) No Child Left Behind Act of 2001 (NCLBA) Provisions Waived Through the Flexibility Initiative; (3) Comments from the Department of Education; and (4) GAO Contact and Staff Acknowledgments.