The Patient Protection and Affordable Care Act (ACA) of 2010 constituted an epic policy breakthrough, culminating a century of effort to ensure that nearly all Americans can obtain health insurance. The articles in this special issue seek to illuminate the challenges involved in implementing this law and other health reforms in a context marked by deep-seated partisan polarization. This introductory essay provides a backdrop to frame the more specific insights of the contributors. It introduces two key legacies in place at the time of the ACA’s passage: the health insurance regime and rising health care costs. Certain pivotal provisions of the ACA then receive attention, including the “individual mandate,” health insurance exchanges, and the Medicaid expansion. This introduction shows how the ACA and related health care developments intersect with broader issues of governance and public administration: the rise of executive branch discretion within the nation's separation of powers system, the increased importance of the administrative presidency, and the emergence of fractious federalism rooted in partisan polarization.