PROJECT SUMMARY One in six children aged 6-17 years had a behavioral health (BH) diagnosis in 2016, with estimates climbing to more than one in four during the COVID-19 pandemic. Limited access to BH services has long presented challenges for most children. Efforts to address the shortcomings of the specialty BH care system, such as integrating BH care into pediatric primary care settings, and state programs that offer telephonic psychiatry consultation to pediatricians, have helped improve care, but immense gaps in care and disparities persist. Accountable care organizations (ACOs) incentivize care integration and population health, which promotes community-health care linkages, care coordination, and interventions to address social determinants of health. The innovative reform introduced by ACOs has the potential to improve BH care delivery, but their impact for pediatric populations is largely unknown. Studies of adult populations suggest that ACOs with certain organizational features (e.g., type of contract) may improve quality of care for chronic diseases but the few studies of the impact of ACOs on quality of care and outcomes for children have not focused on this large, high-risk population of children with BH disorders. This study will be the first to address this critical gap in knowledge by taking advantage of the natural experiment taking place in Massachusetts (MA), a state with a high prevalence of children with BH diagnoses. In 2018, MA, launched 17 new Medicaid ACOs with heterogeneous organizational features (e.g., size; physician vs. hospital-led; age mix. We will leverage MA’s innovative Medicaid ACO ‘experiment’ to address the following aims using mixed methods: 1) Evaluate the impact of ACOs on BH care quality, outcomes, and disparities for Medicaid-insured children; 2) Discern how ACO heterogeneity modifies ACOs’ effects on BH quality and outcomes; 3) Determine the relationship between parent reported experience with their child’s Behavioral Health care and organizational features of the Medicaid ACOs. Fundamental changes in healthcare are needed to address socioeconomic and racial/ethnic disparities in BH care quality and outcomes for children in vulnerable populations. Our innovative mixed methods approach to examining the association between exposure to a new heterogeneous group of Medicaid ACOs and BH care and outcomes uses state-level administrative claims and survey data and key informant interviews with ACO leaders and parents to begin to address this large gap in knowledge. This new information is expected to benefit providers, payers, and policy makers responsible for the care of vulnerable populations of children with BH disorders.