Medicare is a federal program that pays for covered health care services of qualified beneficiaries. It was established in 1965 under Title XVIII of the Social Security Act to provide health insurance to individuals 65 and older, and has been expanded over the years to include permanently disabled individuals under 65. Medicare, which consists of four parts (A-D), covers hospitalizations, physician services, prescription drugs, skilled nursing facility care, home health visits, and hospice care, among other services. Generally, individuals are eligible for Medicare if they or their spouse worked for at least 40 quarters in Medicare-covered employment, are 65 years old, and are a citizen or permanent resident of the United States. Individuals may also qualify for coverage if they are a younger person with a permanent disability, have End-Stage Renal disease (permanent kidney failure requiring dialysis or transplant), or have amyotrophic lateral sclerosis (ALS, Lou Gehrig's disease). The program is administered by the Centers for Medicare & Medicaid Services (CMS), and by private entities that contract with CMS to provide claims processing, auditing, and quality oversight services. In FY2013, the program will cover approximately 52 million persons (43 million aged and 9 million disabled) at a total cost of about $606 billion, accounting for approximately 3.7% of GDP. Spending under the program (except for a portion of administrative costs) is considered mandatory spending and is not subject to the appropriations process. Services provided under Parts A and B (also referred to as "traditional Medicare"), are generally paid directly by the government on a "fee-for-service" basis, using different prospective payment systems or fee schedules. Under Parts C and D, private insurers are paid a monthly "capitated" amount to provide enrollees with at least a minimum standard benefit. Medicare is required to pay for all covered services provided to eligible persons, so long as specific criteria are met. Since 1965, the Medicare program has undergone considerable change. For example, during the 111th Congress, the Patient Protection and Affordable Care Act (ACA; P.L. 111-148 and P.L. 111-152) made numerous changes to the Medicare program that modify provider reimbursements, provide incentives to increase the quality and efficiency of care, and enhance certain Medicare benefits. However, in the absence of further congressional action, the Medicare program is expected to be unsustainable in the long run. The Hospital Insurance (Part A) trust fund has been estimated to become insolvent in 2024. Additionally, although the Supplementary Medical Insurance (Parts B and D) trust fund is financed in large part through federal general revenues and cannot become insolvent, associated spending growth is expected to put increasing strains on the country's competing priorities. As such, Medicare is expected to be a high-priority issue in the 113th Congress, and Congress may consider a variety of Medicare reform options ranging from further modifications of provider payment mechanisms to redesigning the entire program.