The introduction of Medicaid expansions and state Marketplaces under the Affordable Care Act (ACA) have reduced the uninsurance rate to historic lows, changing the choices Americans make about coverage. In this article we shed light on these changing dynamics. We drew upon multistate transition models fit to nationally representative longitudinal data to estimate coverage transition probabilities between major insurance types in the years leading up to and including 2014. We found that the ACA’s unprecedented coverage changes increased transitions to Medicaid and nongroup coverage among the uninsured, while strengthening the existing employer-sponsored insurance system and improving retention of public coverage. However, our results suggest possible weakness of state Marketplaces, since people gaining nongroup coverage were disproportionately older than other potential enrollees. We identified key opportunities for policy makers and insurers to improve underlying Marketplace risk pools by focusing on people transitioning from employer-sponsored coverage; these people are disproportionately younger and saw almost no change in their likelihood of becoming uninsured in 2014 compared to earlier years.