What Is Medicare?

Last reviewed: June 2026 · Reviewed by Galit Sacajiu MD, MPH

Medicare is the federal health insurance program for Americans aged 65 and older, and for younger people with certain disabilities or end-stage renal disease. It was established in 1965 as part of the Social Security Act.

More than 67 million Americans are enrolled in Medicare. The program is run by the Centers for Medicare & Medicaid Services (CMS) and is divided into four parts — each covering different healthcare services.

The Four Parts of Medicare

Who Qualifies for Medicare?

You are eligible for Medicare if you are:

• • A U.S. citizen or permanent legal resident who has lived in the U.S. for at least 5 continuous years, AND
• • Age 65 or older, OR
• • Under 65 with a qualifying disability (receiving Social Security Disability Insurance for 24 months), OR
• • Any age with End-Stage Renal Disease (ESRD) or ALS (Lou Gehrig's Disease)

When to Enroll

Your Initial Enrollment Period (IEP) is the 7-month window around your 65th birthday: 3 months before, the month of, and 3 months after your birthday month.

Missing your IEP without qualifying for a Special Enrollment Period can result in permanent late enrollment penalties — 10% added to your Part B premium for every 12-month period you were eligible but not enrolled.

Original Medicare vs. Medicare Advantage

After enrolling in Medicare, you choose between two main coverage paths:

Frequently Asked Questions