Medicare is a national social insurance program, administered by the U.S. federal government since 1966, that guarantees access to health insurance for Americans aged 65 and older who have worked and paid into the system, and younger people with disabilities as well as people with end stage renal disease and persons with amyotrophic lateral sclerosis. As a social insurance program, Medicare spreads the financial risk associated with illness across society to protect everyone, and thus has a somewhat different social role from for-profit private insurers, which manage their risk portfolio by adjusting their pricing according to perceived risk.

On average, Medicare covers about half (48 percent) of health care costs for enrollees. Medicare enrollees must cover the rest of the cost. These out-of-pocket costs vary depending on the amount of health care a Medicare enrollee needs. They might include uncovered services, such as long-term care, dental, hearing, vision, and supplemental insurance.

To help pay for the costs not covered by Medicare, many people purchase Medicare supplements or Medicare Advantage plans through an agent. To protect seniors from unscrupulous agents, there are strict rules for how to solicit and sell Medicare-related insurance products. Some of these rules may seem ridiculous to young seniors, but they are there to protect, especially those who are experiencing age-related cognitive decline.

There are four "Parts" to Medicare: