Medicare: What It Covers and Who Is Eligible
Medicare is the United States’ federal health insurance program for people age 65 and older and for certain younger people with long-term disabilities or specific diseases. It is administered by the Centers for Medicare & Medicaid Services (CMS). Medicare pays a large share of hospital and medical costs for tens of millions of beneficiaries, but it is not free comprehensive care: premiums, deductibles, coinsurance, and benefit-period rules still apply, and many services (most dental, vision, hearing aids, and long-term custodial care) are limited or excluded under Original Medicare.
This guide explains the four parts of Medicare, eligibility pathways, enrollment windows, Medigap and Medicare Advantage choices, and practical ways to reduce gaps. Dollar amounts for premiums and deductibles change every year. Treat any figure you see in an article or flyer as provisional until you confirm it on Medicare.gov or in your official Annual Notice of Change.
The Four Parts at a Glance
| Part | Common name | Main role |
|---|---|---|
| A | Hospital insurance | Inpatient hospital, limited skilled nursing, hospice, some home health |
| B | Medical insurance | Doctor services, outpatient care, preventive services, durable medical equipment |
| C | Medicare Advantage | Private plans that deliver A and B (usually with extras; often include drugs) |
| D | Prescription drug coverage | Private plans that help pay for outpatient prescription drugs |
Original Medicare means Part A and Part B through the federal program. Medicare Advantage (Part C) replaces the way you receive A and B benefits with a private plan network. Part D can be added to Original Medicare or embedded in many Advantage plans.
Part A — Hospital Insurance
Typical covered services
- Inpatient hospital care (semi-private room, meals, general nursing, drugs given in the hospital as an inpatient)
- Skilled nursing facility (SNF) care after a qualifying inpatient hospital stay, when skilled care is medically necessary—not custodial long-term care
- Hospice care for terminally ill beneficiaries who elect the hospice benefit
- Certain home health services when eligibility rules are met
Benefit periods
Part A cost-sharing is organized around benefit periods, which start when you are admitted as an inpatient and end after a continuous stretch of days out of hospital/SNF care (as defined by Medicare). A new benefit period can mean a new Part A deductible.
Premiums
Most people pay no Part A premium if they or a spouse paid Medicare payroll taxes for about 40 quarters (roughly 10 years). People with fewer quarters may buy Part A by paying a monthly premium.
Important limits
- Part A does not pay for ordinary long-term nursing-home custodial care.
- SNF coverage is limited in days and requires skilled needs; it is not automatic after every hospital visit.
- Observation status versus formal inpatient admission affects eligibility for SNF coverage—ask the hospital how you are classified.
Part B — Medical Insurance
Typical covered services
- Physician and other clinician services
- Outpatient hospital services
- Preventive services and screenings when eligibility criteria are met (many with no Part B cost-sharing when rules are followed)
- Durable medical equipment (DME) such as oxygen equipment or wheelchairs when medically necessary and obtained from enrolled suppliers
- Outpatient mental health services
- Some home health services not covered under Part A
Costs structure
- Monthly Part B premium (standard amount set annually; higher-income beneficiaries pay IRMAA surcharges)
- Annual Part B deductible
- After the deductible, beneficiaries generally pay 20% coinsurance for many Part B services, with no annual out-of-pocket maximum under Original Medicare alone
That absence of a hard cap under Original Medicare is a major reason people buy Medigap or choose Medicare Advantage plans that include maximum out-of-pocket limits.
Part C — Medicare Advantage
Medicare Advantage plans are offered by private insurers approved by Medicare. They must cover Part A and Part B services. Most also include:
- Prescription drug coverage (MA-PD)
- Extra benefits such as routine dental, vision, hearing, fitness, or transportation (varies widely by plan and county)
Plan types commonly include HMOs (stronger network and referral rules) and PPOs (more flexibility, often higher cost out of network). Prior authorization is used more often than in Original Medicare for some services.
Costs
You must continue to pay the Part B premium (and Part A premium if you have one). The Advantage plan may charge an additional premium or advertise $0 plan premium, while using copays and a network. Every Advantage plan has an annual maximum out-of-pocket limit for Part A/B-type services—an important consumer protection compared with bare Original Medicare.
Trade-offs
- Network limits and prior authorization versus broader provider access under Original Medicare
- Extra benefits versus possible constraints on using specialists nationwide
- Plan designs, premiums, and drug formularies change every year, so annual review matters
Part D — Prescription Drug Coverage
Part D is delivered by private plans (stand-alone PDP with Original Medicare, or drug coverage inside many Advantage plans).
What to compare
- Formulary (list of covered drugs) and tier placement
- Pharmacy network and preferred pharmacies
- Deductible, copays, and coinsurance
- Prior authorization, step therapy, and quantity limits
Federal law has been moving Part D toward stronger annual out-of-pocket protections for enrollees; the exact thresholds and phases are updated in CMS rules and plan documents. Always run your actual drug list through the official plan finder rather than relying on a plan’s star rating alone.
Late enrollment penalty
If you go without creditable drug coverage when you are eligible, a permanent penalty can be added to future Part D premiums.
Who Is Eligible
Age 65
U.S. citizens and many long-term lawful permanent residents become eligible at 65. Enrollment timing connects to Social Security/Railroad Retirement status and whether you have employer coverage.
Under 65 with disability
People who receive Social Security Disability Insurance (SSDI) generally become Medicare-eligible after a waiting period (commonly 24 months of entitlement to disability benefits, with important exceptions).
ALS (amyotrophic lateral sclerosis)
Medicare eligibility is accelerated once Social Security disability entitlement related to ALS is established (no full 24-month wait in the usual sense).
End-stage renal disease (ESRD)
People with permanent kidney failure requiring dialysis or transplant can qualify under ESRD rules, with specific enrollment and coordination provisions—including interaction with employer plans and later options to join Medicare Advantage under current law.
Eligibility for premium-free Part A still hinges mainly on work-credit history (or spouse’s record). Others may pay for Part A.
Enrollment Periods You Should Know
Initial Enrollment Period (IEP)
Around the birthday month you turn 65 (a seven-month window spanning three months before, the birth month, and three months after). Missing it can cause gaps and late penalties if you lack other creditable coverage.
Special Enrollment Periods (SEP)
Often available if you delayed Part B because of qualifying group coverage based on current employment, then retire or lose that coverage.
General Enrollment Period
For people who missed earlier windows; coverage start dates and penalties may apply.
Annual Election Period (AEP)
Each fall (typically October 15–December 7), you can switch Advantage plans, switch drug plans, or move between Advantage and Original Medicare (with related Part D choices). Changes usually take effect January 1.
Medicare Advantage Open Enrollment
Early in the year, Advantage enrollees have a limited window to make certain switches (rules are specific—confirm annually).
Working past 65 with job-based insurance requires careful coordination so you do not accidentally incur Part B late penalties.
Medigap (Medicare Supplement) Insurance
Medigap policies are private supplemental plans sold under standardized letter plans in most states (Plan G, Plan N, etc.). They work only with Original Medicare, not with Medicare Advantage.
They typically help pay:
- Part A and B deductibles and coinsurance (depending on the plan letter)
- Some foreign emergency care on certain plans
Medical underwriting
Outside guaranteed-issue windows (for example when you first take Part B at 65 in many situations), insurers in most states may underwrite applications. That makes the first-time Medigap decision strategically important.
Medigap does not include extra dental/vision packages the way many Advantage plans do; those must be purchased separately if desired.
What Medicare Generally Does Not Cover
- Most long-term custodial nursing-home care
- Most routine dental care, eyeglasses, and hearing aids (unless provided as an extra by an Advantage plan or other coverage)
- Care outside the U.S. in most cases (limited exceptions)
- Concierge physician fees not recognized as Medicare-covered services
People with low income should check Medicaid, Medicare Savings Programs, and Extra Help (Part D Low-Income Subsidy).
Tips to Use Medicare Well
- Enroll on time or document employer coverage that allows delay.
- Decide early between Original Medicare + Part D (+ Medigap) versus Medicare Advantage.
- Use Medicare.gov Plan Finder with your drugs and pharmacies each autumn.
- Use preventive benefits—many screenings have no Part B coinsurance when requirements are met.
- Track observation versus inpatient status during hospital stays.
- For high drug costs, compare formularies and ask prescribers about alternatives on lower tiers.
- Contact SHIP (State Health Insurance Assistance Program) for free, unbiased counseling.
- Beware of unsolicited calls selling “Medicare” cards or plans; use official sources.
Dual Eligible Beneficiaries
People with both Medicare and Medicaid may receive help with premiums and cost-sharing and fuller long-term services. Enrollment and plan choices (including Dual Eligible Special Needs Plans in many areas) follow extra rules. Coordinate any plan change with both Medicare and state Medicaid guidance.
Summary
Medicare is a federal program built around Part A (hospital), Part B (medical), optional Part D (drugs), and optional Part C (Medicare Advantage) as an alternative delivery system. Eligibility centers on age 65, long-term disability, ALS, and ESRD pathways. Original Medicare leaves significant cost-sharing and benefit gaps; beneficiaries manage those gaps with Medigap, Advantage plans, low-income programs, or other coverage. Because premiums, deductibles, and plan designs change annually, the most reliable decision process is: confirm eligibility and enrollment timing, compare options with your actual doctors and drugs, and reassess every year during the fall open enrollment period.
This article is educational and not an official CMS determination. Benefit amounts, premiums, and plan rules are updated regularly—verify details on Medicare.gov or with SHIP before enrolling.
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