This report is part of a five-report series on the Individuals with Disabilities Education Act (IDEA) that examines the past-to-current funding levels for all parts of IDEA, analyzes the impact that the lack of full-funding has had on states in meeting their obligations to provide a free and appropriate public education to students with disabilities, and provides recommendations regarding future funding. To better understand how the persistent underfunding of IDEA impacts students with disabilities, the National Council on Disability (NCD) undertook research for this report. Using a mixed methods study, key questions were used to examine the history of the IDEA funding, identifiable impacts in districts and schools resulting from a lack of full funding, and the effect a lack of funding has had on services to students. NCD found that the lack of federal support places considerable pressure on state and local budgets, resulting in a range of actions including one state placing an illegal cap on IDEA identification, districts and schools limiting hiring of personnel and providers, districts and schools restricting service hours, and districts and schools reducing or eliminating other general programs. There is unanimous support to increase annual federal IDEA funds and agreement that increases would allow districts to improve outcomes and use freed-up local funds to support all students. Finally, no large-scale national study has been conducted since the early 2000s, which limits researchers and policymakers' ability to understand the true costs of special education, and maintenance of effort (MOE) requirements can serve as a disincentive to districts piloting innovative or expanded services. To address these findings, NCD recommends the following to Congress, the Department of Education, and state policymakers: (1) Increase funding to the maximum authorized amount; (2) Provide flexibility in IDEA's MOE requirements that also protect student civil rights; and (3) Establish a funding threshold beyond which increases will be tied to improved results for students with disabilities.