What Is Medicare and How Does It Work?

Medicare is a federal health insurance program, mainly for people age 65 and older.

It also covers some younger people who have a qualifying disability or certain medical conditions.

Unlike most employer health insurance, Medicare isn't one policy — it's split into parts, and each part does a different job.

Knowing how those parts fit together is the easiest way to avoid surprise medical bills later.

The Four Parts of Medicare

Here's what each part does and who actually provides it.

For more detail, see our guides to Medicare Part A and Medicare Part B.

Where Medigap Fits

Medicare Supplement Insurance, also called Medigap, is not one of the four parts.

It's a separate private policy that works alongside Original Medicare to help pay costs like deductibles, coinsurance, and copayments.

Medigap does not work with Medicare Advantage — you'd pick one path or the other.

Who Qualifies for Medicare?

Most people become eligible the month they turn 65.

Some people qualify before 65, usually in one of these situations:

• Disability: Medicare generally begins after you've received 24 months of qualifying Social Security or Railroad Retirement Board disability benefits.
• ALS (Lou Gehrig's disease): Medicare can start as soon as SSDI benefits begin, with no 24-month wait.
• End-stage renal disease (ESRD): permanent kidney failure requiring dialysis or a transplant.

People who qualify before 65 often have different enrollment timelines and fewer Medigap options, depending on their state.

Eligibility is based on age or disability, not on whether you've retired — you can qualify for Medicare while you're still working.

Two Ways to Get Your Medicare Coverage

Once you have Part A and Part B, you choose how to receive those benefits.

Original Medicare vs. Medicare Advantage at a Glance

Medicare Advantage rules differ from plan to plan — check a specific plan's network and authorization rules before enrolling.

That missing out-of-pocket limit is one of the details that surprises people most about Original Medicare.

Medigap is one way to protect against those costs; others have retiree coverage from a former employer or help from Medicaid.

If you're weighing Medigap plans, our Medicare Supplement plans comparison chart shows what each plan letter covers.

Example: How Medicare Pays for a Doctor Visit

Suppose you have Original Medicare and see a doctor who accepts Medicare assignment for a covered Part B service.

1. You get care. The doctor sends the claim to Medicare.
2. Medicare pays its share. Once you've met your yearly Part B deductible, Medicare generally pays 80% of the Medicare-approved amount.
3. You owe the rest. You're generally responsible for the remaining 20% coinsurance.
4. Supplemental coverage may step in. If you have a Medigap policy, it may pay some or all of that 20%, depending on the plan.

Does Medicare Cover Everything?

No.

Medicare covers many medically necessary services and preventive benefits, but Original Medicare generally does not cover:

• Most routine dental care and dentures
• Eye exams for prescription glasses
• Hearing aids and exams for fitting them
• Long-term custodial care, such as help with bathing and dressing in a nursing home

You can see the full list on Medicare.gov's page on what Part A and Part B don't cover.

Even for covered services, you'll usually owe deductibles, coinsurance, or copayments unless other coverage picks them up.

Do You Have to Enroll at 65?

Not always — it depends on your situation.

If you're already getting Social Security retirement benefits when you turn 65, you're generally enrolled in Part A and Part B automatically.

If you're not, you'll need to sign up yourself during your Initial Enrollment Period, a seven-month window that starts three months before the month you turn 65.

If you're still working and covered by an employer group health plan, you may be able to delay Part B without a penalty and sign up later during a Special Enrollment Period.

The Social Security Administration explains each enrollment period and when coverage starts.

Don't Miss Your Medigap Window

Your Medigap Open Enrollment Period is a separate six-month window that starts the month you're 65 or older and enrolled in Part B.

During that window, insurers generally can't deny you a Medigap policy or charge you more because of your health.

After it ends, many states let insurers ask health questions, which can make switching or buying a policy harder.

If you're under 65 and on Medicare because of a disability, these federal protections don't automatically apply — your Medigap options depend on your state's rules.

What Should You Do Before Turning 65?

1. Confirm your eligibility and timing. Find your Initial Enrollment Period and check whether you'll be enrolled automatically.
2. Review your current health coverage. If you're still working, ask whether your employer plan lets you delay Part B without a penalty.
3. Choose your path. Decide between Original Medicare (with Part D and possibly Medigap) or a Medicare Advantage plan.
4. Compare plans against your real needs. Check your doctors, prescriptions, expected care, travel habits, and budget.