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Medicare Basics Explained: Parts A, B, C and D in Plain English

Medicare is the federal health insurance program most Americans join around age 65. It is not one single plan — it is a set of parts, each covering a different slice of care. This guide explains who qualifies, what each part does, and the gaps that surprise people most.

Who Qualifies for Medicare?

Most people become eligible for Medicare when they turn 65, as long as they are a US citizen or a legal resident who has lived in the country for at least five years. You do not have to be retired to enroll, and you do not have to stop working — many people join Medicare while still employed, although keeping employer coverage at the same time has its own rules worth checking.

Some people qualify earlier. Medicare also covers younger adults who have received Social Security disability benefits for a set period, and people of any age with end-stage renal disease or ALS. If you already receive Social Security benefits when you turn 65, you are usually enrolled in Parts A and B automatically. If not, you sign up yourself through Social Security.

Part A and Part B: Original Medicare

Part A is hospital insurance. It helps pay for inpatient hospital stays, skilled nursing facility care after a qualifying hospital stay, hospice care, and some home health services. Most people pay no monthly premium for Part A because they — or a spouse — paid Medicare taxes while working. That does not make hospital care free: Part A has a deductible for each benefit period, plus coinsurance for longer stays.

Part B is medical insurance. It covers doctor visits, outpatient care, preventive services such as screenings and flu shots, ambulance services, and durable medical equipment like walkers or blood-sugar monitors. Part B charges a standard monthly premium, which is usually taken out of your Social Security payment, plus an annual deductible. After the deductible, you typically pay 20 percent of the Medicare-approved amount for most services — and there is no yearly cap on that 20 percent under Original Medicare alone.

Together, Parts A and B are called Original Medicare. It is run by the federal government, and you can use it with almost any doctor or hospital in the country that accepts Medicare.

Part C and Part D: Private Plan Options

Part D covers prescription drugs. These plans are sold by private insurance companies approved by Medicare, and each plan has its own list of covered drugs, called a formulary. Even if you take no medicines now, enrolling late without other creditable drug coverage can mean a permanent late-enrollment penalty added to your premium.

Part C, better known as Medicare Advantage, is an alternative way to receive your benefits. Instead of the government paying your providers directly, a private insurer bundles your Part A and Part B coverage — and usually Part D — into one plan, often with extras such as some dental, vision or hearing benefits. In exchange, these plans use provider networks and may require prior authorization for some services. You still pay the Part B premium, and you must stay enrolled in Parts A and B to keep an Advantage plan.

Medicare Advantage vs Medicare Supplement

People with Original Medicare often worry about the open-ended 20 percent coinsurance in Part B. There are two common ways to handle that risk, and they work very differently:

• Medicare Advantage (Part C) replaces the way Original Medicare pays. You use the plan's network and rules, and the plan sets an annual out-of-pocket maximum — something Original Medicare by itself does not have.
• Medicare Supplement (Medigap) works alongside Original Medicare. It is a separate policy that pays some or all of the deductibles and coinsurance that Parts A and B leave behind. You keep the freedom to see any Medicare-accepting provider, but you pay an extra monthly premium, and Medigap policies do not include drug coverage — that is a separate Part D plan.

Neither route is automatically better. The right comparison is your total yearly cost, your doctors, your medicines and how much predictability matters to you.

Enrollment Windows and What Medicare Does Not Cover

Timing matters. Your Initial Enrollment Period runs for seven months around your 65th birthday — starting three months before that month and ending three months after. Missing it can mean late penalties for Part B and Part D that last as long as you have coverage. Each fall, the Annual Enrollment Period (October 15 to December 7) lets you switch between Original Medicare and Advantage plans or change Part D plans for the coming year. Special Enrollment Periods exist for life events such as losing employer coverage or moving.

Just as important is what Medicare leaves out. It generally does not cover:

• Routine dental care and dentures
• Eye exams for glasses, and most vision care
• Hearing aids and routine hearing exams
• Long-term care — extended stays in a nursing home or help with daily activities at home
• Care received outside the United States, except in rare situations

Premiums, deductibles and rules change every year, so treat any dollar figure you see — including in older guides — as a snapshot. For current numbers and your personal enrollment status, the official source is medicare.gov, or 1-800-MEDICARE.

Frequently Asked Questions