Medicare is a national health insurance program in the United States, begun in 1966 under the Social Security Administration (SSA) and now administered by the Centers for Medicare and Medicaid Services (CMS). It provides health insurance for Americans aged 65 and older, younger people with some disability status as determined by the Social Security Administration, as well as people with end stage renal disease and amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease).

In 2018, Medicare provided health insurance for over 59.9 million individuals—more than 52 million people aged 65 and older and about 8 million younger people. On average, Medicare covers about half of healthcare expenses of those enrolled. Medicare is funded by a combination of a payroll tax, beneficiary premiums and surtaxes from beneficiaries, co-pays and deductibles, and general U.S. Treasury revenue.

According to annual Medicare Trustees reports and research by the government's MedPAC group, enrollees almost always cover remaining out-of-pocket costs with additional private insurance, by joining a public Medicare health plan, or both.

Medicare is divided into four Parts. Medicare Part A covers hospital (inpatient, formally admitted only), skilled nursing (only after being formally admitted to a hospital for three days and not for custodial care), and hospice services. Part B covers outpatient services including some providers' services while inpatient at a hospital, outpatient hospital charges, most provider office visits even if the office is "in a hospital", and most professionally administered prescription drugs. Part D covers mostly self-administered prescription drugs. Part C is an alternative called Managed Medicare which allows patients to choose health plans with at least the same service coverage as Parts A and B (and most often more), often the benefits of Part D, and always an annual out-of-pocket spend limit which A and B lack. A beneficiary must enroll in Parts A and B first before signing up for Part C.

In general, all persons 65 years of age or older who have been legal residents of the United States for at least five years are eligible for Medicare. People with disabilities under 65 may also be eligible if they receive Social Security Disability Insurance (SSDI) benefits. Specific medical conditions may also help people become eligible to enroll in Medicare.

People qualify for Medicare coverage, and Medicare Part A premiums are entirely waived, if the following circumstances apply:

  • They are 65 years or older and US citizens or have been permanent legal residents for five continuous years, and they or their spouse (or qualifying ex-spouse) has paid Medicare taxes for at least 10 years.
  • They are under 65, disabled, and have been receiving either Social Security SSDI benefits or Railroad Retirement Board disability benefits; they must receive one of these benefits for at least 24 months from date of entitlement (eligibility for first disability payment) before becoming eligible to enroll in Medicare.

Those who are 65 and older who choose to enroll in Part A Medicare must pay a monthly premium to remain enrolled in Medicare Part A if they or their spouse have not paid the qualifying Medicare payroll taxes.

People with disabilities who receive SSDI are eligible for Medicare while they continue to receive SSDI payments; they lose eligibility for Medicare based on disability if they stop receiving SSDI. The coverage does not begin until 24 month after the SSDI start date. The 24-month exclusion means that people who become disabled must wait two years before receiving government medical insurance, unless they have one of the listed diseases. The 24-month period is measured from the date that an individual is determined to be eligible for SSDI payments, not necessarily when the first payment is actually received. Many new SSDI recipients receive "back" disability pay, covering a period that usually begins six months from the start of disability and ending with the first monthly SSDI payment.

Some beneficiaries are dual-eligible. This means they qualify for both Medicare and Medicaid. In some states for those making below a certain income, Medicaid will pay the beneficiaries' Part B premium for them (most beneficiaries have worked long enough and have no Part A premium), as well as some of their out of pocket medical and hospital expenses.

Medicare has four parts: loosely speaking Part A is Hospital Insurance. Part B is Medical Services Insurance. Medicare Part D covers many prescription drugs, though some are covered by Part B. In general, the distinction is based on whether or not the drugs are self-administered but even this distinction is not total. Public Part C Medicare health plans, the most popular of which are branded Medicare Advantage, are another way for Original Medicare (Part A and B) beneficiaries to receive their Part A, B and D benefits; simply, Part C is capitated fee and Original Medicare is fee for service. All Medicare benefits are subject to medical necessity.

The original program included Parts A and B. Part-C-like plans have existed as demonstration projects in Medicare since the early 1970s, but the Part was formalized by 1997 legislation. Part D was enacted by 2003 legislation and introduced January 1, 2006. Previously coverage for self-administered prescription drugs, if desired, was obtained by private insurance or through a public Part C plan (or by one of its predecessor demonstration plans before enactment).

In April 2018, CMS began mailing out new Medicare cards with new ID numbers to all beneficiaries. Previous cards had ID numbers containing beneficiaries' Social Security numbers; the new ID numbers are randomly generated and not tied to any other personally identifying information.

Part A covers inpatient hospital stays where the beneficiary has been formally admitted to the hospital, including semi-private room, food, and tests. As of January 1, 2018, Medicare Part A had an inpatient hospital deductible of $1340, coinsurance per day as $335 after 61 days confinement within one "spell of illness", coinsurance for "lifetime reserve days" (essentially, days 91-150 of one or more stay of more than 60 days) of $670 per day. The structure of coinsurance in a Skilled Nursing Facility (following a medically necessary hospital confinement of 3 nights in row or more) is different: zero for days 1-20; $167.50 per day for days 21-100. Many medical services provided under Part A (e.g., some surgery in an acute care hospital, some physical therapy in a skilled nursing facility) is covered under Part B. These coverage amounts increase or decrease yearly on 1st day of the year.

The maximum length of stay that Medicare Part A covers in a hospital admitted inpatient stay or series of stays is typically 90 days. The first 60 days would be paid by Medicare in full, except one copay (also and more commonly referred to as a "deductible") at the beginning of the 60 days of $1340 as of 2018. Days 61–90 require a co-payment of $335 per day as of 2018. The beneficiary is also allocated "lifetime reserve days" that can be used after 90 days. These lifetime reserve days require a copayment of $670 per day as of 2018, and the beneficiary can only use a total of 60 of these days throughout their lifetime. A new pool of 90 hospital days, with new copays of $1340 in 2018 and $335 per day for days 61–90, starts only after the beneficiary has 60 days continuously with no payment from Medicare for hospital or Skilled Nursing Facility confinement.

Some "hospital services" are provided as inpatient services, which would be reimbursed under Part A; or as outpatient services, which would be reimbursed, not under Part A, but under Part B instead. The "Two-Midnight Rule" decides which is which. In August 2013, the Centers for Medicare and Medicaid Services announced a final rule concerning eligibility for hospital inpatient services effective October 1, 2013. Under the new rule, if a physician admits a Medicare beneficiary as an inpatient with an expectation that the patient will require hospital care that "crosses two midnights," Medicare Part A payment is "generally appropriate." However, if it is anticipated that the patient will require hospital care for less than two midnights, Medicare Part A payment is generally not appropriate; payment such as is approved will be paid under Part B.[28] The time a patient spends in the hospital before an inpatient admission is formally ordered is considered outpatient time. But, hospitals and physicians can take into consideration the pre-inpatient admission time when determining if a patient's care will reasonably be expected to cross two midnights to be covered under Part A. In addition to deciding which trust fund is used to pay for these various outpatient vs. inpatient charges, the number of days for which a person is formally considered an admitted patient affects eligibility for Part A skilled nursing services.

Part B medical insurance helps pay for some services and products not covered by Part A, generally on an outpatient basis (but also when on an unadmitted observation status in a hospital). Part B is optional. It is often deferred if the beneficiary or his/her spouse is still working and has group health coverage through that employer. There is a lifetime penalty (10% per year on the premium) imposed for not enrolling in Part B when first eligible.

Part B coverage begins once a patient meets his or her deductible ($183 for 2017), then typically Medicare covers 80% of the RUC-set rate for approved services, while the remaining 20% is the responsibility of the patient, either directly or indirectly by private group retiree or Medigap insurance.

Part B coverage includes outpatient physician services, visiting nurse, and other services such as x-rays, laboratory and diagnostic tests, influenza and pneumonia vaccinations, blood transfusions, renal dialysis, outpatient hospital procedures, limited ambulance transportation, immunosuppressive drugs for organ transplant recipients, chemotherapy, hormonal treatments such as Lupron, and other outpatient medical treatments administered in a doctor's office. It also includes chiropractic care. Medication administration is covered under Part B if it is administered by the physician during an office visit.

Part B also helps with durable medical equipment (DME), including but not limited to canes, walkers, lift chairs, wheelchairs, and mobility scooters for those with mobility impairments. Prosthetic devices such as artificial limbs and breast prosthesis following mastectomy, as well as one pair of eyeglasses following cataract surgery, and oxygen for home use are also covered.

Complex rules control Part B benefits, and periodically issued advisories describe coverage criteria. On the national level these advisories are issued by CMS, and are known as National Coverage Determinations (NCD). Local Coverage Determinations (LCD) apply within the multi-state area managed by a specific regional Medicare Part B contractor (which is an insurance company), and Local Medical Review Policies (LMRP) were superseded by LCDs in 2003. Coverage information is also located in the CMS Internet-Only Manuals (IOM), the Code of Federal Regulations (CFR), the Social Security Act, and the Federal Register.

With the passage of the Balanced Budget Act of 1997, Medicare beneficiaries were formally given the option to receive their Original Medicare benefits through capitated health insurance Part C health plans, instead of through the Original fee for service Medicare payment system. Many had previously had that option via a series of demonstration projects that dated back to the early 1970s. These Part C plans were initially known in 1997 as "Medicare+Choice". As of the Medicare Modernization Act of 2003, most "Medicare+Choice" plans were re-branded as "Medicare Advantage" (MA) plans (though MA is a government term and might not even be "visible" to the Part C health plan beneficiary). Other plan types, such as 1876 Cost plans, are also available in limited areas of the country. Cost plans are not Medicare Advantage plans and are not capitated. Instead, beneficiaries keep their Original Medicare benefits while their sponsor administers their Part A and Part B benefits. The sponsor of a Part C plan could be an integrated health delivery system or spin-out, a union, a religious organization, an insurance company or other type of organization.

Public Part C Medicare Advantage and other Part C health plans are required to offer coverage that meets or exceeds the standards set by Original Medicare but they do not have to cover every benefit in the same way (the plan must be actuarially equivalent to Original Medicare benefits). After approval by the Centers for Medicare and Medicaid Services, if a Part C plan chooses to cover less than Original Medicare for some benefits, such as Skilled Nursing Facility care, the savings may be passed along to consumers by offering even lower co-payments for doctor visits (or any other plus or minus aggregation approved by CMS).

Original "fee-for-service" Medicare Parts A and B have a standard benefit package that covers medically necessary care as described in the sections above that members can receive from nearly any hospital or doctor in the country (if that doctor or hospital accepts Medicare). Original Medicare beneficiaries who choose to enroll in a Part C Medicare Advantage or other Part C health plan instead give up none of their rights as an Original Medicare beneficiary, receive the same standard benefits—as a minimum—as provided in Original Medicare, and get an annual out of pocket (OOP) upper spending limit not included in Original Medicare. However they must typically use only a select network of providers except in emergencies or for urgent care while travelling, typically restricted to the area surrounding their legal residence (which can vary from tens to over 100 miles depending on county). Most Part C plans are traditional health maintenance organizations (HMOs) that require the patient to have a primary care physician, though others are preferred provider organizations (which typically means the provider restrictions are not as confining as with an HMO). Others are hybrids of HMO and PPO called HMO-POS (for point of service) and a few public Part C health plans are actually fee for service hybrids.

Medicare Part D went into effect on January 1, 2006. Anyone with Part A or B is eligible for Part D, which covers mostly self-administered drugs. It was made possible by the passage of the Medicare Modernization Act of 2003. To receive this benefit, a person with Medicare must enroll in a stand-alone Prescription Drug Plan (PDP) or public Part C healh plan with integrated prescription drug coverage (MA-PD). These plans are approved and regulated by the Medicare program, but are actually designed and administered by various sponsors including charities, integrated health delivery systems, unions and health insurance companies; almost all these sponsors in turn use pharmacy benefit managers in the same way as they are used by sponsors of health insurance for those not on Medicare. Unlike Original Medicare (Part A and B), Part D coverage is not standardized (though it is highly regulated by the Centers for Medicare and Medicaid Services). Plans choose which drugs they wish to cover (but must cover at least two drugs in 148 different categories and cover all or "substantially all" drugs in the following protected classes of drugs: anti-cancer; anti-psychotic; anti-convulsant, anti-depressants, immuno-suppressant, and HIV and AIDS drugs). The plans can also specify with CMS approval at what level (or tier) they wish to cover it, and are encouraged to use step therapy. Some drugs are excluded from coverage altogether and Part D plans that cover excluded drugs are not allowed to pass those costs on to Medicare, and plans are required to repay CMS if they are found to have billed Medicare in these cases.

Under the 2003 law that created Medicare Part D, the Social Security Administration offers an Extra Help program to lower-income seniors such that they have almost no drug costs; in addition approximately 25 states offer additional assistance on top of Part D. It should be noted again for beneficiaries who are dual-eligible (Medicare and Medicaid eligible) Medicaid may pay for drugs not covered by Part D of Medicare. Most of this aid to lower-income seniors was available to them through other programs before Part D was implemented.

Coverage by beneficiary spending is broken up into four phases: deductible, initial spend, gap (infamously called the "donut hole"), and catastrophic. Under a CMS template, there is usually a $100 or so deductible before benefits commence (maximum of $415 in 2019) followed by the initial spend phase where the templated co-pay is 25%, followed by gap phase (where originally the templated co-pay was 100% but that will fall to 25% in 2020 for all drugs), followed by the catastrophic phase with a templated co-pay of about 5%. The beneficiaries' OOP spend amounts vary yearly but are approximately as of 2018 $1000 in the initial spend phase and $3000 to reach the catastrophic phase. This is just a template and about half of all Part D plans differ (for example, no initial deductible, better coverage in the gap) with permission of CMS, which it typically grants as long as the sponsor provides at least the actuarial equivalent value.

No part of Medicare pays for all of a beneficiary's covered medical costs and many costs and services are not covered at all. The program contains premiums, deductibles and coinsurance, which the covered individual must pay out-of-pocket. A study published by the Kaiser Family Foundation in 2008 found the Fee-for-Service Medicare benefit package was less generous than either the typical large employer preferred provider organization plan or the Federal Employees Health Benefits Program Standard Option. Some people may qualify to have other governmental programs (such as Medicaid) pay premiums and some or all of the costs associated with Medicare.

Medicaid in the United States is a federal and state program that helps with medical costs for some people with limited income and resources. Medicaid also offers benefits not normally covered by Medicare, including nursing home care and personal care services. The Health Insurance Association of America describes Medicaid as "a government insurance program for persons of all ages whose income and resources are insufficient to pay for health care."Medicaid is the largest source of funding for medical and health-related services for people with low income in the United States, providing free health insurance to 74 million low-income and disabled people (23% of Americans) as of 2017. It is a means-tested program that is jointly funded by the state and federal governments and managed by the states, with each state currently having broad leeway to determine who is eligible for its implementation of the program. States are not required to participate in the program, although all have since 1982. Medicaid recipients must be U.S. citizens or qualified non-citizens, and may include low-income adults, their children, and people with certain disabilities. Poverty alone does not necessarily qualify someone for Medicaid.

The Patient Protection and Affordable Care Act (PPACA) significantly expanded both eligibility for and federal funding of Medicaid. Under the law as written, all U.S. citizens and qualified non-citizens with income up to 133% of the poverty line, including adults without dependent children, would qualify for coverage in any state that participated in the Medicaid program. However, the Supreme Court of the United States ruled in National Federation of Independent Business v. Sebelius that states do not have to agree to this expansion in order to continue to receive previously established levels of Medicaid funding, and many states have chosen to continue with pre-ACA funding levels and eligibility standards.

Research suggests that Medicaid improves recipients' financial security. However, the evidence is mixed regarding whether Medicaid actually improves health outcomes, although "the best existing evidence says [having health insurance] improves health".

Medicaid and Medicare are the two government sponsored medical insurance programs in the United States and are administered by the U.S. Centers for Medicare & Medicaid Services in Baltimore, Maryland.

Medi-Cal is California's Medicaid health care program. This program pays for a variety of medical services for children and adults with limited income and resources. Medi-Cal is supported by Federal and state taxes. Medi-Cal is a large program made up of many separate programs designed to assist Californians in various family and medical situations.

California Medi-Cal requirements: In order to qualify for this benefit program, you must be a resident of the state of California, in need of health care/insurance assistance, and whose financial situation would be characterized as low income or very low income. You must also be a U.S. national, citizen, or have satisfactory immigration status. To qualify for this benefit, you must:

  • Be over the age of 64; or
  • Be pregnant or have a child 18 or under; or
  • Be blind or disabled; or
  • Have a child, parent, or spouse in your household who is blind or disabled