Most people become eligible for Medicare at 65. You may also qualify earlier if you've received Social Security disability benefits for two years, or if you have ESRD (kidney failure) or ALS. If you're already drawing Social Security before 65, you're usually enrolled in Parts A and B automatically; otherwise you sign up through Social Security.

Timing matters because Medicare runs on windows:

• Initial Enrollment Period — a seven-month window around your 65th birthday (the three months before, your birthday month, and the three months after).
• Special Enrollment Period — if you kept coverage through a current job past 65, you generally get up to eight months to enroll after that employment or coverage ends, without penalty.
• General Enrollment Period — January 1 to March 31, a fallback if you missed your window.
• Open Enrollment — October 15 to December 7 each year, to review or change Medicare Advantage and drug plans for the coming year.

Missing the window you're supposed to use can mean a lifelong late-enrollment penalty, so it's worth confirming which one applies to you.

Medicare comes in "parts," and you don't have to memorize them — here's the simple version:

• Part A is hospital insurance: inpatient hospital stays, skilled nursing after a qualifying hospital stay, hospice, and some home health. Most people get Part A premium-free.
• Part B is medical insurance: doctor visits, outpatient care, lab and imaging, medical equipment, and preventive services.
• Part D is prescription drug coverage, offered through private plans.

Parts A and B together are called Original Medicare. From there you have two ways to round it out: add a Part D drug plan (and often a Medigap supplement) to Original Medicare, or choose a Part C "Medicare Advantage" plan, which bundles A and B — and usually D — into one private plan. The rest of this series walks through each piece so you can see how they fit your situation.

Part B is the "doctor and outpatient" half of Original Medicare. It helps pay for office visits, outpatient care, lab tests and imaging, mental health services, durable medical equipment like walkers and wheelchairs, and a long list of preventive services — many of which cost you nothing when the provider accepts Medicare assignment.

Part B has a monthly premium set each year by Medicare; higher-income households can pay an income-related surcharge on top. There's also an annual deductible, after which you typically pay a share of the cost (coinsurance) for covered services.

The thing to get right with Part B is timing. If you don't have coverage through a current job, you're generally expected to enroll when you first turn 65 — waiting can trigger a lifelong late-enrollment penalty. If you do have active employer coverage, you may be able to delay, but confirm first, because the rules depend on your employer's size.

Part D is prescription drug coverage, sold by private insurers that Medicare approves. You can get it as a stand-alone drug plan paired with Original Medicare, or built into most Medicare Advantage plans.

A few things shape what you'll actually pay:

• The formulary — each plan covers its own list of drugs and sorts them into tiers, with lower tiers usually costing less.
• The pharmacy network — you generally pay least at the plan's preferred pharmacies, and some plans only cover in-network pharmacies.
• Plan rules — some drugs require prior authorization or step therapy, or have quantity limits.

Because every plan's drug list and pharmacy network is different, the "best" Part D plan is simply the one that covers your specific medications at the lowest total cost — and that can change from year to year. One timing rule to know: if you go 63 days or more without Part D or other creditable drug coverage after you're first eligible, you can owe a late-enrollment penalty for as long as you have Part D.

Medicare Advantage (Part C) is an all-in-one alternative to Original Medicare, offered by private insurers Medicare approves. One plan gives you your Part A and Part B coverage, and most plans include Part D drug coverage too.

• Networks and rules. Advantage plans (HMOs, PPOs, and others) use provider networks and may require referrals or prior authorization, so your access and costs depend on the plan.
• A yearly out-of-pocket limit. Every Advantage plan caps what you pay out of pocket for covered Part A and B services each year — something Original Medicare alone doesn't have.
• Costs. You generally keep paying your Part B premium, plus any plan premium (many are $0) and copays as you use care.
• Extras. Many plans add benefits Original Medicare doesn't, like limited dental, vision, or hearing — but the details vary by county.

One important rule: you can't pair a Medigap supplement with a Medicare Advantage plan. It's one path or the other.

Medigap — also called Medicare Supplement — is insurance you add to Original Medicare to help pay your share of the costs: deductibles, copays, and coinsurance. You need both Part A and Part B to buy one.

• It works with Original Medicare, not instead of it, so you keep nationwide access to any provider that accepts Medicare. You can't use Medigap with a Medicare Advantage plan.
• Plans are standardized and labeled by letter (A through N), so the same letter offers the same core benefits no matter which company sells it — which makes them easier to compare.
• Medigap doesn't include drug coverage, so you'd add a separate Part D plan.
• Some plans include limited foreign-travel emergency coverage.

Timing is important: your best chance to buy is the six-month Medigap Open Enrollment Period that starts when you're 65 and enrolled in Part B. During that window you can't be turned down or charged more for health reasons. After it ends, insurers can often use medical underwriting, and the rules vary by state.

This is the choice most people wrestle with: Original Medicare with a Medigap supplement and a Part D drug plan, or a Medicare Advantage plan. Neither is universally "better" — they're built differently.

Original Medicare + Medigap + Part D gives you the widest provider choice (any doctor or hospital that accepts Medicare, nationwide) and predictable cost-sharing, since Medigap covers most of your out-of-pocket costs. The trade-off is separate premiums, and extras like dental or vision aren't built in.

Medicare Advantage gives you one plan for A and B, usually with Part D included, plus a yearly out-of-pocket cap and often extras like dental or vision — frequently with a low or $0 plan premium. The trade-off is that you use the plan's network and follow its rules, and benefits differ by county.

A simple way to decide: if broad provider access and predictable costs matter most, lean Medigap; if you're comfortable with a network and like an all-in-one plan with extras, lean Advantage. Your doctors, prescriptions, travel, and budget all factor in.

The two paths cost money in different shapes, which is why comparing only premiums can mislead you.

With Medicare Advantage, you often pay a low or $0 plan premium, then pay copays and coinsurance as you actually use care — doctor visits, procedures, hospital stays — up to the plan's yearly out-of-pocket maximum. Your cost rises with how much care you use, but it can't exceed that cap.

With Original Medicare plus Medigap, you pay a higher fixed monthly premium (Part B, the Medigap policy, and a Part D plan), but Medigap then covers most of your cost-sharing — so what you pay when you use care is small and predictable.

So the real question isn't "which premium is lower" — it's "which total cost fits my situation." Someone who rarely needs care might pay less overall with Advantage; someone who wants steady, predictable costs, or who uses a lot of care, often prefers Medigap. Your health, the drugs you take, how much you travel, and your tolerance for surprise bills all tip the balance. There's no single cheapest plan — there's the one that fits you.