The Affordable Care Act (ACA) called for every state in the United States (US) to significantly 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. We will use electronic health record data (EHR) from the OCHIN practice-based research network, which serve >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, we 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, building on our prior work, we will link EHR data from the 213 OCHIN clinics in Oregon to Medicaid administrative claim 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. We will study insurance coverage, healthcare utilization, and Medicaid expenditures addressing 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.