For 50 years, Medicare has accomplished its two key goals: ensure access to health care for its elderly and disabled beneficiaries, and protect them against the financial hardship of health care costs. As the single largest source of health insurance in the United States, with 55 million covered through the program, Medicare has both shaped the U.S. health system and responded as needs have demanded. But rising costs, affecting the federal budget and beneficiaries, are an ongoing challenge that will need to be addressed. As the Medicare population continues to age, better strategies also are required to serve the increasing number of beneficiaries with complex care needs and chronic conditions. To control Medicare's costs while enhancing the quality of its coverage and care, policymakers must continue to identify and spread promising payment and delivery system reforms that can help the program achieve success over the next 50 years.