Medicaid provides health insurance to tens of millions of Americans, including many who had no coverage before the Affordable Care Act allowed states to extend eligibility for the program. In a new Commonwealth Fund study, David Blumenthal, M.D., Petra Rasmussen, Sara R. Collins, and Michelle M. Doty investigate how well Medicaid does its job by measuring whether beneficiaries have adequate access to care, good-quality care, and protection against financial hardship.