What is Medicare? A Complete Beginner's Guide

Who qualifies for Medicare

Most people qualify when they turn 65, but eligibility is not strictly age-based. There are three primary paths:

• Age 65 or older. You qualify if you or your spouse worked for at least 10 years (40 quarters) in Medicare-covered employment, and you are a U.S. citizen or permanent legal resident who has lived in the country for at least five consecutive years.
• Under 65 with a qualifying disability. Most people who receive Social Security Disability Insurance (SSDI) qualify for Medicare after 24 months of SSDI benefits.
• End-Stage Renal Disease or ALS. People with permanent kidney failure requiring dialysis or a transplant, or with amyotrophic lateral sclerosis, may qualify regardless of age and without the 24-month waiting period.

The four parts of Medicare

Medicare is structured in four parts. Each part covers a different category of services and follows its own rules for enrollment, premiums, and out-of-pocket costs.

Part A — Hospital insurance

Part A covers inpatient hospital stays, skilled nursing facility care after a qualifying hospital stay, hospice, and some home health services. Most people pay no premium for Part A because they earned it through working years of Medicare-taxed employment. Part A still has deductibles and coinsurance for inpatient stays.

Part B — Medical insurance

Part B covers physician visits, outpatient services, preventive care, durable medical equipment, mental health services, and many other medically necessary services. Part B has a monthly premium that the federal government sets each year, plus an annual deductible and 20% coinsurance for most services after the deductible is met. Higher-income beneficiaries may pay an additional amount called IRMAA.

Part C — Medicare Advantage

Part C is an alternative way to receive Medicare benefits through a private insurance company contracted with Medicare. Medicare Advantage plans bundle Part A and Part B coverage, and most include Part D drug coverage. Plans typically use provider networks and have their own cost-sharing structures, which vary by plan and area. Medicare Advantage is one option; Original Medicare with optional supplements is the other. Neither is universally better — the right approach depends on your situation.

Part D — Prescription drug coverage

Part D is prescription drug coverage offered through private plans approved by Medicare. You can get Part D as a standalone plan paired with Original Medicare, or built into many Medicare Advantage plans. Each plan has its own formulary (list of covered drugs), pharmacy network, and cost structure. There is a late enrollment penalty if you go without creditable drug coverage when first eligible.

When you can enroll

Medicare has specific enrollment windows. Missing them can result in delayed coverage and lifetime late enrollment penalties for Part B and Part D.

• Initial Enrollment Period (IEP). A seven-month window that begins three months before the month you turn 65 and ends three months after.
• General Enrollment Period (GEP). January 1 through March 31 each year, for people who missed their IEP and don't qualify for a Special Enrollment Period.
• Special Enrollment Period (SEP). Triggered by qualifying events like losing employer coverage. Each SEP has its own timing and documentation requirements.
• Annual Enrollment Period (AEP). October 15 through December 7 each year. Existing Medicare beneficiaries can change Medicare Advantage or Part D plans during this window.

Original Medicare vs. Medicare Advantage

Original Medicare refers to Parts A and B run directly by the federal government. Medicare Advantage (Part C) is the alternative through which a private plan delivers Part A and B coverage on Medicare's behalf, often with additional features. The two paths differ in provider access rules, cost predictability, and how additional coverage like supplements or drug plans is layered on. People often add a Medigap (Medicare Supplement) policy and a standalone Part D plan to Original Medicare; Medicare Advantage typically bundles drug coverage and may include other elements that vary by plan and area. Choosing between the two is one of the most consequential Medicare decisions and depends on factors like provider preferences, travel patterns, predictable-cost priorities, and budget.

What Medicare doesn't cover

Original Medicare does not cover routine dental care, routine vision care, hearing aids, most long-term care, cosmetic procedures, or care received outside the United States in most cases. Some of these gaps can be addressed with separate insurance products like standalone dental and vision policies. Medicare Advantage plans may include certain extras, which vary by plan and area. Reviewing what you actually use and want covered helps inform which path makes sense.

Costs and what you pay

Medicare is not free. Even when Part A has no premium, you pay deductibles, coinsurance, and copayments. Part B has a standard premium that can be higher for higher-income beneficiaries. Part D plans have premiums, deductibles, and cost-sharing that vary by plan. Medigap policies, standalone Part D plans, and Medicare Advantage plans may each have separate premiums. Total cost depends on your choices, your income, and your usage. Working with a licensed advisor can help model costs against your specific situation.