Since the enactment of P.L. 94-142, the predecessor legislation to the Individuals with Disabilities Education Act (IDEA), in 1975, the federal government has played a prominent role in encouraging the principle of educational equality for children with disabilities through a permanent, broad-scale federal assistance program. The IDEA is a grants statute that provides federal funding for the education of children with disabilities and requires, as a condition for the receipt of such funds, that states agree to provide a free appropriate public education (FAPE; i.e., specially designed instruction provided at no cost to the parents that meets the needs of a child with a disability) to every eligible child. The IDEA, most recently reauthorized by P.L. 108-446 in 2004, was appropriated approximately $13.4 billion in FY2018. The largest part of the IDEA is Part B, Assistance for Education of all Children with Disabilities, which covers special education for children and youth with disabilities between the ages of 3 and 21. Approximately 92% of total IDEA appropriations fund the Part B, Section 611, grants-to-states program. This report examines the development of the allocation formula for the Part B grants-to-states program, the major changes to the formula over the past 40 years, current funding levels and trends, and how allocations are currently calculated. Issues concerning the funding of special education and related services are also discussed.