Medicare is the national social insurance program administered by the federal government. It guarantees access to health insurance for Americans ages 65 and older, as well as young people with disabilities and those with end-stage renal disease. As a social insurance program, Medicare spreads the financial risk associated with illness across society to protect everyone, therefore playing a different role than private insurers.

In 2010, the program provided health insurance to 48 million Americans, including 40 million people age 65 and older and 8 million younger people with disabilities. On average, Medicare covers about half of healthcare costs for enrollees. The program was founded in 1965, under Title XVIII of the Social Security Act to provide health insurance to people 65 or older, regardless of income or medical history. Prior to its creation, only half of those over 65 had health insurance, with coverage either unavailable or unaffordable to those without.