There are millions of older Americans who rely on Medicare for health coverage. And if you’re retiring this year, you may be gearing up to enroll as well.

But it’s important to know the ins and outs of how Medicare works. With that in mind, here are some key things to know about Medicare if you’re about to retire.

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1. You’re generally not eligible until age 65

The earliest age you can sign up for Social Security is 62. Because of this, you might assume that Medicare coverage will become available to you at that same age.

But actually, Medicare eligibility doesn’t usually begin until age 65 (there can be exceptions for people with certain medical conditions). So if your plan is to retire at 62 and sign up for Social Security and Medicare at the same time, you’ll need to rethink it.

2. You don’t have to sign up for both Parts A and B

Many people sign up for Medicare Parts A and B when they turn 65. Part A covers hospital care, and Part B covers outpatient care.

But a key difference between the two is that most enrollees are not charged a monthly premium for Part A, whereas Part B has one. For this reason, you may want to consider enrolling in Part A alone when you turn 65 if you have the option to stay on a group health plan.

Say you’re retiring this year, but you get your group health coverage through a spouse’s job that’s not coming to an end anytime soon. What you could do is take Part A only and have it serve as secondary insurance. This way, you won’t be paying for Part B coverage you don’t need.

3. If you’re losing your qualifying group health coverage, it pays to enroll on time

If you have health coverage through a qualifying group plan, which generally means a plan with 20 employees or more, then it often makes sense to hold off on Medicare Part B unless you’re not happy with your employer plan. But if you’re retiring and losing that coverage, and you’re old enough for Medicare, then it pays to sign up during the seven-month initial enrollment window around your 65th birthday.

There can be steep penalties for enrolling in Medicare late, and those penalties could haunt you for the rest of your retirement. So if you won’t qualify for a special enrollment period due to being on a qualifying group health plan, then you should make sure to sign up during your initial enrollment period.

4. You shouldn’t forget about Medigap if you’re sticking with original Medicare

Medicare enrollees have a choice to stick with Parts A and B plus a Part D drug plan, or get comprehensive coverage through Medicare Advantage. If you choose the former option, then it pays to sign up for Medigap (supplemental insurance as well). Otherwise, you could face large out-of-pocket costs under original Medicare.

Your initial window to sign up for Medigap starts the month you’re 65 or older and have Medicare Part B. During this time, you can’t be denied coverage for pre-existing conditions and are likely to secure the most favorable rates.

These are only some of Medicare’s rules it pays to be informed about as you gear up for retirement. If you’re ending your time in the workforce this year, make a point to read up on Medicare first so you’re able to go in prepared.

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