The expansion of the Medicaid program under the Affordable Care Act (ACA) is one of the most consequential and controversial changes in the history of US health policy. During the struggle over ACA implementation, attention has largely focused on partisan disagreements about whether states should expand Medicaid coverage. The states have also been the site of a quieter struggle to modernize Medicaid’s managed care programs, in which many new Medicaid beneficiaries will be required to enroll.