SUMMARY People with intellectual or developmental disabilities (IDD)—a sizeable and growing population—now live longer and enjoy better quality of life than in prior decades. However, this group still suffers from excess morbidity, especially during adolescence and young adulthood, the time when they are expected to make health care transitions (HCTs) from child-centered to adult-oriented health care systems. Although it is likely that some adolescents and young adults with IDD experience high-quality HCTs, our understanding of how HCTs proceed longitudinally, in vivo for large populations of adolescents and young adults with IDD is extremely limited. At a population level, we do not know the ages at which HCTs start or end or whether the timing depends on the type of physicians involved. Additionally, scant data exist regarding how health care quality changes for this population during HCTs and the role insurance gaps may play in disrupting that care quality. The proposed study combines three recently developed claims-based tools to create one of the largest, longitudinal, and multi-payer datasets of adolescents and young adults with IDD to date. It uses five years of all-payer claims databases (2014–2018) in three states to identify individuals aged 10–28 with IDD (69,000 persons in Colorado, 85,000 persons in Massachusetts, and 217,000 persons in New York). It then pursues the following four Aims: (1) to characterize HCTs for adolescents and young adults with IDD in terms of the types of physicians involved and the ages across which transfers occur; (2) to assess the quality of care received by people with IDD during the ages at which HCTs occur; (3) to characterize insurance gaps associated with Medicaid age 19 eligibility rules; and (4) to examine the relationship between insurance gaps following Medicaid’s eligibility redetermination at age 19 and clinical care quality using quasi-experimental methods. This study will establish much-needed basic facts about how HCTs proceed for adolescents and young adults with IDD so that doctors, nurses, and allied health professionals can help develop delivery system, insurance, and payment policy interventions that better create health among adolescents and young adults with IDD. It is novel in the database it creates for study, the features of HCTs that it will quantify, its assessment of care quality, and its use of quasi-experimental methods to examine the relationship between insurance gaps around the time of Medicaid’s age 19 eligibility rules and the health of adolescents and young adults with IDD. This research addresses the NICHD’s strategic priority to “improve the transition from adolescent to adult health care…for [those] with disabilities” and the Intellectual and Developmental Disabilities Branch’s priorities to “understand the complexity of comorbid symptoms [of IDD]” and “promote…treatments for IDD that will impact clinical care.”