What Is Medicare Insurance and How Does It Work?

Medicare is a federal health insurance program that covers most Americans aged 65 and older, along with certain younger people with disabilities or specific medical conditions. It is divided into distinct parts, each handling a different slice of medical care, and it is funded through a mix of payroll taxes, general tax revenue, and premiums paid by the people it covers. Understanding how these parts fit together, what they cost, and where the gaps are can save you real money and prevent frustrating surprises when you need care.

The Four Parts of Medicare

Medicare is not a single insurance plan. It is a collection of coverage programs, each designated by a letter, each with its own rules, costs, and scope.

Part A (Hospital Insurance): Covers inpatient hospital stays, stays in skilled nursing facilities after a qualifying hospital admission, hospice care, and some home health visits. Most people do not pay a monthly premium for Part A if they or a spouse paid Medicare payroll taxes for at least ten years.
Part B (Medical Insurance): Covers outpatient services like doctor visits, preventive screenings, lab tests, durable medical equipment, and outpatient surgeries. Part B requires a monthly premium, which in a standard year runs around $175 or more depending on your income.
Part C (Medicare Advantage): An alternative way to receive your Part A and Part B benefits through a private insurance company rather than directly from the federal government. Medicare Advantage plans often bundle additional benefits like dental, vision, and hearing coverage. Enrollment in these plans has grown substantially over the years, with access and availability closely tied to how the government pays the private insurers.
Part D (Prescription Drug Coverage): Covers outpatient prescription medications. Part D is offered through private insurance companies, either as a standalone drug plan paired with Original Medicare or as part of a Medicare Advantage plan. Recent legislation has significantly changed how Part D works, including the introduction of a $2,000 annual cap on out-of-pocket drug spending.

Parts A and B together are often called “Original Medicare” or “Traditional Medicare.” You can stick with Original Medicare and add a Part D drug plan, or you can opt into a Part C Medicare Advantage plan that wraps everything together. That choice shapes nearly every aspect of your healthcare experience, from which doctors you can see to how much you pay out of pocket.

Who Qualifies

The most common path into Medicare is turning 65. If you are a U.S. citizen or permanent resident who has lived in the country for at least five continuous years and you or your spouse paid Medicare payroll taxes during your working life, you are eligible for premium-free Part A at 65. Part B is available to the same group, though you pay a monthly premium for it regardless of work history.

People younger than 65 can also qualify. If you receive Social Security Disability Insurance benefits, you become eligible for Medicare, but not immediately. There is a required 24-month waiting period after your disability benefits begin before Medicare coverage kicks in.¹ That two-year gap has been controversial for decades, and Congress has repeatedly considered proposals to shorten or eliminate it. During that waiting period, people often rely on employer coverage, COBRA, marketplace plans, or Medicaid if they qualify.

Two exceptions skip the waiting period entirely. People diagnosed with end-stage renal disease who need dialysis or a kidney transplant can get Medicare coverage relatively quickly. And those diagnosed with amyotrophic lateral sclerosis (ALS) receive Medicare as soon as their disability benefits begin, with no waiting period at all.

How Medicare Is Paid For

Medicare’s total spending reached about $1 trillion in 2023, and the money comes from three main streams: payroll taxes, general federal revenue, and beneficiary premiums.² Each part of Medicare draws on a different mix of these sources.

Part A runs primarily on payroll taxes. Employers and employees each pay 1.45 percent of earnings, for a combined 2.9 percent tax. In 2023, payroll taxes accounted for about 88 percent of Part A’s revenue.² This money flows into the Hospital Insurance Trust Fund, which is the pot of money that pays for inpatient care.

Part B leans heavily on general revenues from the U.S. Treasury, which covered about 71 percent of Part B costs in 2023, with beneficiary premiums covering roughly 27 percent.² Part D follows a similar pattern: general revenues fund about 73 percent, beneficiary premiums cover 14 percent, and state payments for people who qualify for both Medicare and Medicaid make up the remaining 12 percent.²

What this means practically is that Parts B and D cannot technically go “bankrupt” the way Part A can, because their funding is set annually by Congress and the Treasury. Part A, on the other hand, depends on a trust fund that could run short if payroll tax collections do not keep pace with hospital spending.

What Medicare Does Not Cover and Why Supplemental Insurance Matters

Original Medicare has real gaps. It does not cover most dental care, routine eye exams, hearing aids, or long-term custodial care in a nursing home. It also leaves you responsible for deductibles, copayments, and coinsurance that can add up quickly, especially during a hospitalization or a serious illness.

Because of these gaps, most people on Medicare carry some form of supplemental coverage. About 20 percent of beneficiaries purchase a Medigap policy, which is a private insurance plan designed specifically to cover Original Medicare’s cost-sharing requirements like deductibles and coinsurance. Others get supplemental protection through retiree health benefits from a former employer, through Medicaid if their income is low enough, or through Medicare Advantage plans that typically offer more comprehensive benefits than Original Medicare alone. Only about 10 percent of Medicare beneficiaries have no additional coverage at all.³

Medigap plans are standardized and labeled by letter (Plan G, Plan N, and so on), and each letter offers a specific set of benefits. They can only be paired with Original Medicare, not with Medicare Advantage. If you choose Medicare Advantage instead, your plan typically has its own out-of-pocket maximum built in, which serves a similar protective function.

Medicare Advantage vs. Original Medicare

The choice between Original Medicare and Medicare Advantage is the biggest structural decision you make in Medicare. In Original Medicare, the federal government pays your doctors and hospitals directly. You can see any provider in the country who accepts Medicare, and you have no network restrictions. In Medicare Advantage, a private insurer receives a monthly payment from Medicare on your behalf and manages your care through a plan that usually has a provider network, referral requirements, and its own cost-sharing rules.

Medicare Advantage plans often include extras that Original Medicare does not, such as dental and vision benefits, gym memberships, and over-the-counter drug allowances. But they also come with trade-offs. One of the most significant is prior authorization, a process where the insurer must approve certain services before you receive them. A cross-sectional study of Medicare Advantage insurers found that among the roughly 14,000 clinical services covered under Part B, insurers required prior authorization for anywhere from about 950 to nearly 3,000 of them, depending on the insurer. On a weighted average, about 23 percent of Part B spending and 18 percent of service use required prior authorization across the five largest Medicare Advantage insurers studied.⁴ Original Medicare does not use prior authorization for Part B services, which means fewer bureaucratic hurdles but also fewer checks on unnecessary spending.

Historically, the availability and generosity of Medicare Advantage plans has fluctuated with how much the government pays insurers. When payments to plans go up, more insurers enter the market and enrollment grows. When payments tighten, plans pull out of certain regions or reduce benefits.⁵ Today, more than half of all Medicare beneficiaries are enrolled in Medicare Advantage, a dramatic shift from two decades ago when it was a niche option.

How Outcomes Compare Between the Two

A reasonable question is whether Medicare Advantage actually delivers better or worse care than Original Medicare. The research paints a mixed but generally favorable picture for Advantage plans on several measures, with caveats.

A large study comparing postacute care found that Medicare Advantage beneficiaries spent about six fewer days in skilled nursing facilities and nearly four fewer days in home health care after a hospital discharge than their counterparts in Original Medicare. They also had a lower probability of hospital readmission (by about 1.5 percentage points), spent more days living in the community during the first 100 days after discharge, and showed modestly better functional recovery.⁶ A separate analysis of full-risk Medicare Advantage plans found that 16 of 20 quality measures favored the Advantage group, including lower inpatient admissions, fewer emergency department visits, and better medication adherence, while the remaining four measures were statistically equivalent.⁷ Earlier research found consistently lower emergency department use and lower overall resource use among Medicare Advantage enrollees.⁸

These numbers look good for Medicare Advantage, but they come with an important asterisk. Medicare Advantage plans tend to attract healthier enrollees on average, and while researchers use statistical methods to adjust for this, it is difficult to fully account for. The lower utilization could reflect more efficient care coordination, or it could partly reflect that the plans attract people who need less care to begin with, or it could reflect the prior authorization barriers that discourage some service use. The honest answer is that it is probably a combination of all three, with the relative weight of each factor still debated.

Part D and the New Out-of-Pocket Cap

Prescription drug coverage under Part D has gone through its biggest overhaul since the program launched in 2006. The Inflation Reduction Act included several changes effective in 2024 and 2025, the most significant being a $2,000 annual cap on out-of-pocket drug costs for Part D enrollees.⁹ Before this cap, beneficiaries who took expensive specialty medications could face thousands of dollars in cost-sharing after passing through a coverage gap sometimes called the “donut hole.”

The law also shifted a larger share of drug spending from the federal government to the private plan sponsors that administer Part D. The idea is to give insurers a stronger incentive to negotiate lower drug prices, since they now bear more of the financial risk. Whether this restructuring will actually lower drug costs or simply lead to narrower formularies and tighter restrictions remains to be seen, and early evidence on plan design changes is still emerging.⁹

If you are healthy at 65 and wonder whether enrolling in Part D right away is worth it, the financial math favors signing up. Part D carries a late enrollment penalty: for every month you delay past your initial eligibility without creditable drug coverage, you pay a permanent surcharge on your premium. Research modeling the lifetime costs found that under current penalty rules, immediate enrollment is the better financial choice for most beneficiaries, even if you take few medications at the time you turn 65.¹⁰

When You Qualify for Both Medicare and Medicaid

About 12 million Americans are “dual eligible,” meaning they qualify for both Medicare and Medicaid simultaneously. This typically happens when someone is 65 or older (or has a qualifying disability) and also has income low enough to meet their state’s Medicaid thresholds. Dual eligibility can be tremendously valuable because Medicaid picks up costs that Medicare does not, including long-term nursing home care, dental services in many states, and Medicare premiums and cost-sharing.

The challenge is that Medicare and Medicaid are run by different entities (federal government and state governments, respectively), and the two programs were not designed to work together seamlessly. Coordinating care across two separate systems leads to fragmented coverage, duplicated paperwork, and confusion for both patients and providers. Special Needs Plans, a type of Medicare Advantage plan designed specifically for dual-eligible beneficiaries, were created to address this problem. But research has found that these plans have not dramatically expanded enrollment in coordinated Medicare-Medicaid products, and they need contractual relationships with state Medicaid programs to offer meaningful benefits beyond what regular Medicare Advantage provides.¹¹

Several integrated care models have been tried, including Programs of All-Inclusive Care for the Elderly (PACE), Medicare-Medicaid Plans, and Fully Integrated Dual Eligible Special Needs Plans. A systematic review evaluated how these programs performed on spending, utilization, care coordination, and patient experience compared to non-integrated arrangements.¹² Results were mixed across programs, reinforcing the difficulty of merging two bureaucracies into a single smooth experience. If you are dual eligible, it is worth checking whether your state offers one of these integrated plans, because when they work well, they reduce the administrative burden on you and can improve access to services that neither program covers well on its own.

What Medicare Has Meant for Older Americans’ Finances

Before Medicare existed, roughly half of Americans over 65 had no health insurance at all, and medical bills were a leading cause of poverty in old age. The program’s impact on financial protection has been dramatic and well documented. Research on Medicare’s initial decade found that while the program had no detectable effect on mortality rates in those early years, it produced a substantial reduction in financial risk. Out-of-pocket medical spending for those in the top quartile of the spending distribution fell by about 40 percent after Medicare was introduced.¹³

That financial protection continues today. Research examining what happens when people reach 65 and gain Medicare eligibility found that out-of-pocket spending drops by about a third at the mean and by more than half at the 95th percentile of spending. Medical-related financial strain, including difficulty paying bills and contact from collections agencies, drops sharply at the same threshold.¹⁴ In other words, Medicare’s most consistent and measurable benefit has always been protecting people from financial catastrophe rather than extending life spans per se.

Value-Based Care and Accountable Care Organizations

Medicare has increasingly moved away from simply paying doctors and hospitals for every service they perform (known as fee-for-service) and toward models that reward quality and efficiency. Accountable Care Organizations are one of the most prominent examples. In an ACO, a network of primary care physicians, specialists, hospitals, and rehabilitation facilities agrees to coordinate care for a defined group of Medicare patients. If the group meets quality benchmarks while keeping total spending below a target, the providers share in the savings.¹⁵

Early evidence from the Medicare Shared Savings Program has shown that ACOs have achieved some success in both improving care quality and reducing costs, though the results have been uneven across participating organizations.¹⁶ For you as a beneficiary, being attributed to an ACO does not restrict your provider choices in Original Medicare the way a network does in Medicare Advantage. You may not even realize you are part of one. The goal is for the coordination to happen in the background, with providers communicating more effectively about your care, reducing duplicated tests, and catching problems earlier.

The Trust Fund Solvency Question

You may have heard that Medicare is “going bankrupt.” This claim is both overstated and grounded in a real concern, and the distinction matters. The Part A Hospital Insurance Trust Fund does face projected insolvency, meaning there is a date in the future when the fund’s income from payroll taxes will no longer be enough to cover all Part A claims. One analysis pegged that date at 2028 and emphasized that reforms would be needed to cover beneficiaries’ care.¹⁷ The latest Medicare Trustees Report has since pushed the depletion date back to 2036, reflecting more recent spending trends and economic conditions.¹⁸

This projected insolvency date has been moving around for decades. An analysis of Trustees Reports from 1985 through 2024 found that the insolvency date was extended 20 times over that 40-year span. The Trustees estimated in 2012 that the fund would be completely exhausted by now, and obviously that did not happen. Major legislative reforms like the Balanced Budget Act of 1997 and the Affordable Care Act of 2010 had larger and more sustained impacts on solvency than originally estimated, in part because they constrained provider payments and indirectly slowed spending growth across the program.¹⁸

Insolvency does not mean Medicare disappears. If the trust fund were depleted without any Congressional action, Part A could still pay a substantial share of claims from ongoing payroll tax revenue; it just could not pay all of them. Parts B and D, as noted earlier, draw from general revenues and cannot become insolvent in the same way. Politically, letting Medicare hospital coverage shrink overnight would be so untenable that Congress has historically acted before depletion actually occurs. The risk is not that Medicare vanishes but that the fixes involve some combination of higher taxes, reduced provider payments, increased beneficiary costs, or changes to eligibility. The specific path depends entirely on which political coalition controls Congress at the time.

1
Policy Archive. Social Security Disability Insurance (SSDI) and Medicare: The 24-Month Waiting Period for SSDI Beneficiaries Under Age 65
2
KFF. FAQs on Medicare Financing and Trust Fund Solvency
3
JAMA Internal Medicine. The Future of Medicare Supplemental Insurance
4
PubMed Central. Comparison of prior authorization across insurers: cross sectional evidence from Medicare Advantage
5
PubMed Central. An economic history of Medicare part C
6
JAMA Network Open. Postacute Care Use and Outcomes Among Medicare Advantage vs Traditional Medicare Beneficiaries
7
PubMed. Health outcomes under full-risk Medicare Advantage vs traditional Medicare
8
PubMed Central. A Comparison of Relative Resource Use and Quality in Medicare Advantage Health Plans Versus Traditional Medicare
9
PubMed Central. Changes in Medicare Part D Plan Designs After the Inflation Reduction Act
10
PubMed. Should healthy Medicare beneficiaries postpone enrollment in Medicare Part D?
11
PubMed Central. Special Needs Plans and the coordination of benefits and services for dual eligibles
12
PubMed Central. Quality, Spending, Utilization, and Outcomes Among Dual-Eligible Medicare-Medicaid Beneficiaries in Integrated Care Programs: A Systematic Review
13
Journal of Public Economics. What did Medicare do? The initial impact of Medicare on mortality and out of pocket medical spending
14
American Economic Journal: Economic Policy. The Effects of Medicare on Medical Expenditure Risk and Financial Strain
15
PubMed Central. The role of accountable care organizations in delivering value
16
Journal for Healthcare Quality. Benchmarking Implications: Analysis of Medicare Accountable Care Organizations Spending Level and Quality of Care
17
PubMed Central. Options to Extend Medicare’s Trust Fund: Lessons From Japan’s Statutory Health Insurance
18
Health Affairs Scholar. Solvency extensions to the Medicare Hospital Insurance Trust Fund: what is driving them?