The Patient Protection and Affordable Care Act (ACA) expands access to health insurance in the United States, and, to date, an estimated 20 million previously uninsured individuals have gained coverage. Understanding the law's impact on coverage, access, utilization, and health outcomes, especially among low-income populations, is critical to informing ongoing debates about its effectiveness and implementation. Early findings indicate that there have been significant reductions in the rate of uninsurance among the poor and among those who live in Medicaid expansion states. In addition, the law has been associated with increased health care access, affordability, and use of preventive and outpatient services among low-income populations, though impacts on inpatient utilization and health outcomes have been less conclusive. Although these early findings are generally consistent with past coverage expansions, continued monitoring of these domains is essential to understand the long-term impact of the law for underserved populations.