Understanding Medicare: Your Complete Guide

Medicare is one program in four pieces, and people usually assemble two or three of them.

• Part A — hospital insurance. Inpatient hospital stays, skilled nursing facility care after a qualifying hospital stay, hospice, and some home health care. Most people pay no premium for Part A because they or a spouse paid Medicare taxes for at least 40 quarters. It is not free of cost sharing: the inpatient deductible is $1,736 per benefit period in 2026, and a benefit period can start more than once in a year.
• Part B — medical insurance. Doctor visits, outpatient care, preventive services, durable medical equipment and mental health care. The standard premium is $202.90 a month in 2026, with a $283 annual deductible, after which Medicare generally pays 80% of the approved amount and you pay the rest — with no yearly cap on your 20%.
• Part C — Medicare Advantage. A private plan Medicare approves to deliver your Part A and Part B benefits, usually with drug coverage built in. You stay in Medicare and keep paying the Part B premium. Plans set their own networks and cost sharing, and each must set a yearly in-network out-of-pocket maximum.
• Part D — prescription drug coverage. Sold by private insurers under Medicare's rules, either as a standalone plan alongside Original Medicare or inside an Advantage plan. Each plan has its own drug list, so the same prescription can be covered differently from one plan to the next.

"Original Medicare" means Parts A and B on their own. From there people either add a Medicare Supplement (Medigap) policy and a Part D plan, or take a Medicare Advantage plan instead.

They solve the same problem — the share of the bill Original Medicare leaves you — in opposite ways, and choosing between them is the decision most people find hardest.

A Medicare Advantage plan replaces the way you receive your benefits. Care generally runs through the plan's own network and service area, some services need a referral or prior authorization, and you pay copays as you use care. In exchange, every plan sets a yearly in-network out-of-pocket maximum, which Original Medicare does not have, and many plans include drug coverage in the same package. What a specific plan covers, and what it charges, is in that plan's own documents — it varies by county and changes each January.

A Medicare Supplement (Medigap) policy sits alongside Original Medicare and pays some or all of the coinsurance, copayments and deductibles Medicare leaves. You keep access to any provider in the country who accepts Medicare, with no network and no referrals, and you add a separate Part D plan for prescriptions. Medigap benefits are standardized by law, so a Plan G covers the same things at every company; price and service are what differ.

The timing detail that costs people the most: the six-month Medigap open enrollment window, when a company cannot refuse you or price you up for your health, starts the month you are both 65 or older and enrolled in Part B. Switching from Advantage to Medigap later can mean answering health questions.

A Medigap policy pays after Medicare pays, on Medicare-covered services only. The exact gaps it fills depend on the letter: the benefits are standardized, so the chart is the same at every company and you can read a plan off it row by row.

The rows that matter most in practice are the Part A hospital coinsurance (every letter covers it, plus 365 extra days after Medicare's own days run out), the Part B coinsurance — your 20% — which most letters cover in full, the Part A deductible, skilled nursing coinsurance, and Part B excess charges, which arise when a doctor does not accept Medicare assignment.

Two rules narrow the field. Plans that pay the annual Part B deductible are closed to anyone who became eligible for Medicare on or after January 1, 2020, which is why Plan G rather than Plan F is the fullest plan most new enrollees can buy. And no Medigap policy sold today includes prescription drug coverage, so a Medigap household still needs a Part D plan.

The full standardized chart, letter by letter, is on the Medicare Supplement hub.

Part D is prescription drug coverage sold by private insurers under Medicare's rules, and you get it either as a standalone plan alongside Original Medicare or built into a Medicare Advantage plan.

Costs come in layers and every layer varies by plan: a monthly premium, a deductible of up to $615 in 2026, then copays or coinsurance at the pharmacy. Once your spending on covered drugs reaches $2,100 in 2026 you pay nothing more for covered drugs that year — a recent change that puts a ceiling on the worst case. Covered insulin is capped at $35 for a month's supply by federal law, with no deductible.

The part people regret getting wrong is timing. Going 63 days or more without Part D or other creditable drug coverage after your Initial Enrollment Period ends adds 1% of the national base beneficiary premium ($38.99 in 2026) for every full uncovered month, for as long as you have drug coverage. Coverage from an employer or union plan, VA benefits or TRICARE usually counts as creditable — your plan must tell you in writing each year.

The details, including how to challenge a drug that is not on the list, are on the Part D hub.

Six windows do almost all the work.

• Initial Enrollment Period. Seven months: the three before the month you turn 65, that month, and the three after. Enroll in the first three and coverage starts the first of your birthday month; enroll later and it starts the first of the month after you sign up.
• Annual Enrollment Period. October 15 to December 7 every year. Switch between Original Medicare and Medicare Advantage, change Advantage plans, or change or drop a Part D plan. What you choose takes effect January 1.
• Medicare Advantage Open Enrollment. January 1 to March 31, for people already in an Advantage plan: one change, either to a different Advantage plan or back to Original Medicare.
• Special Enrollment Periods. Opened by a qualifying event — most commonly employer coverage ending, which opens an eight-month window for Part B. COBRA and retiree coverage do not count as current employment.
• General Enrollment Period. January 1 to March 31 for people who missed their Initial Enrollment Period and have no Special Enrollment Period. Coverage starts the first of the month after you enroll, and a Part B late-enrollment penalty may apply.
• Medigap open enrollment. Six months from the month you are both 65 or older and enrolled in Part B.

To see your own dates, use the Initial Enrollment Period calculator; every window is explained in full on the enrollment hub.