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For most people, the answer is during your Initial Enrollment Period — the 7-month window that starts 3 months before the month you turn 65 and ends 3 months after. Signing up in the 3 months before your birthday month lets coverage start the month you turn 65.

The big exception is if you're still working past 65 with employer coverage (more on that below). Missing your window without other creditable coverage can mean a lifelong late-enrollment penalty.

Check your exact enrollment window →

It depends mostly on how many employees your employer has:

• 20 or more employees: Your group plan is usually primary. You can often delay Part B without penalty and enroll later through a Special Enrollment Period (you get 8 months after the job or coverage ends). Many people still take premium-free Part A — but if you contribute to an HSA, enrolling in any part of Medicare stops new HSA contributions, so check first.
• Fewer than 20 employees: Medicare is usually primary, so most people should enroll in Part A and Part B at 65 to avoid gaps and penalties.

Compare your group plan's cost and coverage against Medicare — sometimes Medicare plus a Supplement is actually cheaper.

Use the "Do I need Part B?" guide →

A Medicare Supplement (Medigap) works alongside Original Medicare to pay much of what Medicare leaves behind. You can see any doctor that accepts Medicare nationwide, premiums are higher, and out-of-pocket costs are very predictable.

Medicare Advantage (Part C) replaces Original Medicare with a private plan, often with a $0 or low premium and extras like dental or vision — but you use a network and there's an annual out-of-pocket maximum you could reach in a bad year.

Neither is "better" — it depends on your doctors, prescriptions, travel, and budget. That's exactly the conversation a good agent has with you.

See a full side-by-side comparison →

For 2026, the standard Part B premium is $202.90/month with a $283 annual deductible. Most people pay $0 for Part A; the Part A hospital deductible is $1,736 per benefit period. Higher-income households pay more for Part B and Part D through an income surcharge called IRMAA.

On top of that, you'll have the premium for whatever coverage you choose (a Supplement, an Advantage plan, and/or a Part D drug plan).

Estimate your 2026 premium →

During your 6-month Medigap Open Enrollment Period — which starts when you're 65 or older and enrolled in Part B — you have a "guaranteed issue" right, meaning you can't be turned down or charged more for health reasons. Outside that window, in most states, insurers can use medical underwriting. This is one big reason timing matters.

Each fall, the Annual Enrollment Period runs October 15 – December 7 — you can change your Medicare Advantage or Part D drug plan, with changes effective January 1. If you're already in an Advantage plan, the Medicare Advantage Open Enrollment Period (January 1 – March 31) lets you switch once or return to Original Medicare.

Your MSIS Inc. agent reviews your plan every year to make sure it still fits — that's the "service after the sale" part.

No. Your premium is set by the insurance carrier and is the same whether you enroll on your own or with an agent. Agents are compensated by the carriers, so our help — the education, the comparison, and the service afterward — costs you nothing.