evidence: At the ten year mark since the passage of the Affordable Care Act (ACA), a substantial body of research has investigated effects of the Medicaid expansion on coverage; access to care and related measures (including utilization, quality of care and health outcomes, provider capacity, and affordability and financial security); and various economic measures. This issue brief summarizes findings from 404 studies (including 80 newly included since the last update of this analysis) of the impact of state Medicaid expansions under the ACA published beginning in January 2014 (when the coverage provisions of the ACA went into effect) and updates earlier versions of this brief with studies through January 2020.1 [...] Coverage: Studies show that Medicaid expansion states experienced significant coverage gains and reductions in uninsured rates among the low-income population broadly and within specific vulnerable populations. States that implemented the expansion with a waiver have seen coverage gains, but some waiver provisions appear to compromise coverage. Access to care and related measures: Most research demonstrates that Medicaid expansion has improved access to care, utilization of services, the affordability of care, and financial security among the low-income population. Studies show improved self-reported health following expansion and an association between expansion and certain positive health outcomes. A small subset of study findings showed no effects of expansion on certain specific measures within these access-related categories. Findings on expansion’s effect on provider capacity are mixed, with studies showing increases, decreases, or no effects on measures like appointment availability or wait times.