Numbers of People with Intellectual or Developmental Disabilities Known to State Developmental Disabilities Agencies in the United States, FY 2015 State IDD agencies were asked to report how many people were known to their agency. This could include anyone who receives IDD services, people waiting for IDD services, and people known to the state IDD agency, but who are not receiving any services. It does not include people with IDD who receive services provided by another state agency. 49 states reported the total number of people with IDD they served or tracked and 48 states reported those numbers separately for children and youth (ages birth to 21 years) and adults (22 years or older). The age distribution of people served in each state is very important to consider when examining state differences in living arrangements and Medicaid Waiver expenditures for people with IDD. This project is funded through a cooperative agreement from the Administration on Community Living, U.S. Department of Health and Human Services Grant #90DN0297 with supplemental support from the National Institute on Disability and Independent Living Rehabilitation Research Grant #H133B130006. Grantees undertaking projects under government sponsorship are encouraged to express freely their findings and conclusions. Points of view or opinions do not therefore necessarily represent official ACL or NIDRR policy.