 The Affordable Care Act ACA called for every state in the United States US to expand Medicaid coverage to individuals at 138 of the federal poverty level by 2014. In a 2012 legal challenge, the Supreme Court ruled that states were not required to implement the ACA-sponsored Medicaid expansion; thus by December 31, 2014, only 27 states and the District of Columbia expanded Medicaid, while 23 states did not. Little is known about the impact of ACA-sponsored Medicaid expansions on access to and changes in receipt of healthcare services among vulnerable populations. Yet this knowledge is essential to informing deliberations in non-expansion states regarding whether to expand their Medicaid programs, and will also be helpful to expansion states eager to learn about the impact of their expansions. The natural experiment created by the Supreme Court decision to make expansion optional for states provides a unique opportunity to assess the extent to which Medicaid expansions improve access to healthcare for low-income patients and other vulnerable populations. The proposed project will be one of the first to examine changes in access to and receipt of healthcare services and to Medicaid expenditures following ACA Medicaid expansions among a large population of safety net clinic patients. The investigators will use electronic health record data EHR from the OCHIN practice-based research network, which serves 1 million patients in 442 community health centers CHCs in 8 expansion states and 8 non-expansion states. Moreover, using CHC data from the 8 expansion-states, the investigators will examine differences in utilization among patients gaining new Medicaid coverage newly insured, as compared to those already insured by Medicaid already insured, and those who remain uninsured uninsured. Finally, the investigators will link EHR data from the 213 OCHIN clinics in Oregon to Medicaid administrative claims data to assess overall healthcare utilization including care received outside of CHCs, such as hospitalization and to compare Medicaid expenditures among newly insured individuals versus those already insured. The investigators will address the following specific aims: Aim 1: compare pre-post health insurance status, primary care, mental health, and dental visits, and receipt of preventive services, as well as changes in payer mix among OCHIN CHCs in states that did and did not expand Medicaid; Aim 2: Examine pre-post utilization of CHC services including receipt of preventive services by newly insured patients compared to already insured patients and uninsured patients; and, Aim 3: Measure pre-post Medicaid expansion changes in overall utilization of healthcare services and costs to the Oregon Medicaid program among newly insured compared to already insured. The findings from this project will be extremely relevant to policy and practice, informing further improvements in the US healthcare system to ensure access to healthcare for vulnerable populations. 