US Healthcare · System Basics
Medicare Basics
Medicare eligibility, Parts A through D, Medigap supplements, enrollment windows, late enrollment penalties, and how to verify a provider accepts Medicare.
People approaching age 65, individuals with qualifying disabilities, and family members helping someone navigate Medicare enrollment and coverage options.
Medicare is the federal health insurance program primarily serving people aged 65 and older, as well as certain younger individuals with qualifying disabilities or conditions. Administered by the Centers for Medicare and Medicaid Services (CMS), Medicare covers a broad range of healthcare services but involves premiums, deductibles, and copays — it is not free coverage. Understanding the different parts of Medicare, enrollment rules, and the options for supplemental coverage helps you make informed decisions at each stage of eligibility.
Medicare eligibility and who qualifies
The most common path to Medicare eligibility is turning 65. US citizens and lawful permanent residents who have lived in the US for at least five continuous years are eligible. You do not need to be retired — you can have Medicare while still working.
Younger people may also qualify for Medicare if they have received Social Security Disability Insurance (SSDI) payments for a period defined by federal rule. Additionally, individuals with certain conditions — end-stage renal disease (ESRD) requiring dialysis or a kidney transplant, or amyotrophic lateral sclerosis (ALS) — may qualify at any age.
If you already receive Social Security retirement benefits when you turn 65, you will generally be enrolled in Parts A and B automatically. If you are not yet receiving Social Security benefits, you must actively enroll during your Initial Enrollment Period — a seven-month window surrounding your 65th birthday.
The four parts of Medicare
Part A — Hospital insurance. Part A covers inpatient hospital care, skilled nursing facility (SNF) care following a qualifying hospital stay, hospice care, and some home health services. Most people do not pay a premium for Part A because they (or a spouse) paid Medicare taxes during their working years. Part A has deductibles and cost-sharing that apply per benefit period.
Part B — Medical insurance. Part B covers outpatient services, physician visits, preventive care, mental health services, and durable medical equipment (DME). Part B requires a monthly premium, which is set by CMS and adjusted annually. There is also an annual deductible, and after that, Part B typically covers a defined percentage of the approved amount for covered services.
Part C — Medicare Advantage. Medicare Advantage plans are offered by private insurers approved by Medicare and must cover everything that Original Medicare covers. Most Part C plans also include prescription drug coverage (Part D) and may offer additional benefits such as dental, vision, or hearing. Enrolling in Part C means your benefits are delivered through the private plan rather than directly through the federal government. These plans typically have provider networks, so the provider you want to see must participate in the plan.
Part D — Prescription drug coverage. Part D provides prescription drug coverage through standalone plans or through Medicare Advantage plans that include drug coverage. Part D plans have formularies — lists of covered drugs organized into tiers — with different cost-sharing at each tier. Plans vary significantly in which drugs they cover and at what cost. Enrolling in a Part D plan when first eligible and maintaining creditable drug coverage avoids late enrollment penalties.
Medigap supplemental insurance
Original Medicare (Parts A and B) leaves beneficiaries responsible for deductibles, coinsurance, and copays that can be substantial. Medigap (also called Medicare Supplement Insurance) policies, sold by private insurers, are designed to pay some or all of these remaining costs.
Medigap plans are standardized by federal law into lettered plan types (Plan G, Plan N, etc.), each with a defined set of benefits. You pay a monthly premium to the private insurer in addition to your Part B premium. Medigap plans do not include prescription drug coverage — you would need a separate Part D plan.
The best time to purchase a Medigap policy is during your Medigap open enrollment period, which begins when you turn 65 and enroll in Part B. During this window, insurers cannot deny you coverage or charge more based on your health status. Outside this window, medical underwriting may apply.
Late enrollment penalties
Missing enrollment deadlines for Parts B and D can result in permanent premium penalties. For Part B, the penalty is a percentage increase per full 12-month period you were eligible but not enrolled, with exceptions if you had qualifying employer coverage. For Part D, the penalty is calculated monthly and added to your premium for as long as you have Medicare drug coverage.
Exceptions exist when you delay enrollment because you have creditable coverage elsewhere — such as through an employer plan or a spouse’s plan. When you lose that coverage, a Special Enrollment Period allows you to enroll without penalty. Documenting your creditable coverage period is important.
Verifying provider participation
Not all providers accept Medicare. Providers fall into three categories: participating (accepts Medicare assignment for all patients), non-participating (can see Medicare patients but may charge more), and opted-out (does not accept Medicare at all). The Medicare Care Compare tool at medicare.gov lets you search for providers who accept Medicare in your area.
Key terms
| Term | Plain meaning | Glossary |
|---|---|---|
| Medicare Part A | Covers inpatient hospital stays, skilled nursing facility care, hospice, and some home health | → |
| Medicare Part B | Covers outpatient services, doctor visits, preventive care, and durable medical equipment | → |
| Medicare Part C | Medicare Advantage — private plans that deliver Medicare benefits and often include Part D | → |
| Medicare Part D | Prescription drug coverage, available as a standalone plan or through Medicare Advantage | → |
| Medicare Supplement | Medigap — private insurance that helps pay costs Original Medicare does not fully cover | → |
Common questions
- When does Medicare eligibility begin?
- Medicare eligibility generally begins at age 65 for US citizens and lawful permanent residents who have lived in the US for at least five years. People under 65 may also qualify if they have received Social Security Disability Insurance for a defined period or have certain qualifying conditions.
- Is Part A free?
- Most people do not pay a premium for Part A if they or their spouse worked and paid Medicare taxes for a sufficient period as defined by Social Security rules. People who do not meet the work requirement can buy into Part A coverage at a monthly premium set by CMS.
- What is the difference between Original Medicare and Medicare Advantage?
- Original Medicare (Parts A and B) is administered directly by the federal government and accepted by most providers nationwide. Medicare Advantage (Part C) is administered by private insurers approved by Medicare; it often includes drug coverage and may offer additional benefits, but typically restricts you to a specific network of providers.
- What happens if I miss my initial enrollment window?
- Missing the initial enrollment period for Part B or Part D can result in a late enrollment penalty — a permanent increase to your monthly premium. The penalty amount is calculated based on how long you were eligible but not enrolled. Exceptions apply if you had creditable coverage from another source.
- Can I have both Medicare and Medicaid?
- Yes. People who qualify for both programs are called dual eligible. Medicaid may cover costs that Medicare does not, including premiums, copays, and services not covered by Medicare. Coordination of benefits determines which program pays first.
Sources
Last reviewed: September 2026