How Medicare Works

What Medicare is — and isn't

Medicare is a federal health insurance program administered by the Centers for Medicare & Medicaid Services (CMS). It is not the same as Medicaid, which is a separate joint federal-state program primarily based on income, though some people qualify for both.

Medicare is generally available to people age 65 and older who meet work-history or citizenship/residency requirements, and to some younger people who have received Social Security disability benefits for a required period or who have specific conditions such as end-stage renal disease. Eligibility rules have detail beyond this overview, so anyone unsure of their status should confirm directly with Social Security or Medicare.gov.

The program does not automatically cover every medical need. Understanding what is covered, what is not, and where the gaps are is the core of using Medicare well.

The building blocks: Parts A, B, C and D

Medicare is organized into parts rather than a single unified plan. Part A generally addresses inpatient hospital, skilled nursing facility, hospice and some home health care. Part B generally addresses outpatient medical care — doctor visits, preventive services, durable medical equipment and outpatient procedures.

Together, Parts A and B are often called 'Original Medicare.' On top of that foundation, people choose whether to add a stand-alone Part D prescription drug plan, or to replace Original Medicare altogether with a Part C Medicare Advantage plan sold by a private insurer under contract with CMS.

A separate guide in this series, Medicare Parts A, B, C and D Explained, walks through each part in more depth.

Two paths to getting covered

Once someone is eligible, they generally choose between two structural paths. The first is Original Medicare (Parts A and B) administered directly by the federal government, often paired with a stand-alone drug plan and sometimes a supplemental (Medigap) policy to help with the costs Original Medicare does not cover.

The second path is Medicare Advantage (Part C), where a private insurer manages the benefit on Medicare's behalf, typically bundling hospital, medical and often drug coverage into one plan with its own network and cost-sharing rules.

Neither path is universally 'better' — they trade off differently on provider flexibility, predictability of costs, extra benefits and plan structure. The comparison guide in this series covers those tradeoffs in detail.

Why enrollment timing matters

Medicare uses defined enrollment windows rather than open, anytime sign-up. There is an Initial Enrollment Period tied to turning 65 (or reaching Medicare-eligible disability status), a General Enrollment Period for those who missed their initial window, an Annual Open Enrollment Period for changing certain existing coverage, and Special Enrollment Periods triggered by specific life events such as losing employer coverage.

Missing a window can matter in two ways: coverage may not start when needed, and in some cases delayed enrollment can lead to a late-enrollment adjustment to future premiums. The exact windows, dates and any adjustment mechanics are set by CMS and can change, so this series intentionally describes the structure rather than specific dates or amounts. Confirm your personal timeline at Medicare.gov or with the Social Security Administration.

See the companion guide, When to Enroll in Medicare, for a closer look at each enrollment window.

What Original Medicare typically does not fully cover

Original Medicare generally involves cost-sharing — amounts the beneficiary pays such as premiums, deductibles and coinsurance — rather than covering care at no cost. It also does not automatically include most dental, vision, hearing or long-term custodial care.

This is why many beneficiaries add a stand-alone drug plan, a Medigap supplemental policy, or choose a Medicare Advantage plan that may bundle extra benefits. Which combination fits depends on health needs, budget tolerance for unpredictable costs, and provider preferences.

How Medicare is paid for

Medicare is funded through a mix of payroll taxes, general federal revenue, and premiums paid by beneficiaries. Beneficiaries themselves may owe premiums, deductibles and coinsurance depending on which parts and plans they use, and some premiums can be higher for people with higher income.

Because these dollar figures are updated annually by CMS, this guide deliberately does not state specific premium, deductible or coinsurance amounts. The current figures are published each year at Medicare.gov.

Where to verify current details

Medicare's rules, costs and plan availability are reviewed and adjusted on a regular cycle. Before making a decision, it is worth checking Medicare.gov directly, calling 1-800-MEDICARE, or contacting your State Health Insurance Assistance Program (SHIP) for free, unbiased counseling.

This guide series is designed to help you understand how the pieces fit together conceptually so that when you do look up current numbers, you know what they mean and how they affect your choices.