Normally, you’re required to enroll in Medicare at age 65, or else pay higher premiums when you do enroll later. However, there is one caveat to this rule: Those who are still working, and covered by an employer’s healthcare policy, might be able to delay Medicare enrollment without incurring the penalty.

With more people over 65 continuing to work, this issue has taken on increased importance. Here’s what you need to know if you’re still employed when you reach your 65th birthday.

The size of your employer matters. If your company employs 20 or more workers, and you’re enrolled in their group health insurance plan, you can delay your Medicare enrollment. This is called creditable coverage and you’re exempt from the requirement to enroll in Medicare at 65.

However, you might wish to enroll in Medicare Part A anyway. Depending upon your work history, you’re probably eligible to enroll without a premium. In the event that you need a hospital stay, Medicare Part A will act as secondary insurance to back up your group benefits. At the time you apply, you can delay Part B (which does charge a premium) due to having creditable coverage.

If you work for a company with fewer than 20 employees, you must enroll in Medicare even if you’re covered by a group health insurance policy. Your Medicare (Parts A and B) will serve as your primary insurance, and your group health plan will be secondary.

If your group healthcare plan includes prescription drug coverage, you might be able to delay Medicare Part D without a penalty.

These rules apply to those with working spouses, too. If it’s not you, but your spouse who is employed, these same rules apply if you’re covered on their group healthcare plan.

Your future enrollment process will be a bit different. One day, you will likely retire, and at that point coverage under your employer’s group healthcare plan will end. Now will you enroll in Medicare like other retirees. If you claim Social Security benefits you will be automatically enrolled in Medicare.

If you’re not claiming Social Security yet, enroll in Medicare Part A by visiting ssa.gov. You will likely be eligible for this coverage without a premium, based on your work history.

Part B is a bit more tricky. When your employment ends, you will qualify for a Special Enrollment Period beginning on the day of your separation and lasting for eight months. This should give you plenty of time to sign up for Part B. You should file Forms CMS-40B and CMS-L564 with Social Security, to certify your enrollment and your exemption from the late enrollment penalty (due to prior creditable coverage).

For other types of Medicare coverage, consult with an experienced insurance professional. If you wish to enroll in Medicare Part C (Advantage plans), Part D (prescription coverage), or a Medigap policy, many considerations must be taken. Your insurance professional can help you understand all of the options available to you, and guide you through the enrollment process.