If you are covered by a group health plan with 20 or more employees, that plan is your primary insurer until you (or your spouse if the source of your coverage) retire or otherwise lose coverage. This means that you do not need to sign up for Medicare until the group health plan coverage ends, at which time you will have a special enrollment period when you can enroll in Medicare, a Medigap or Medicare Advantage plan, and Part D (prescription) plan. However, if your coverage comes through a company with fewer than 20 employees, your group health plan is usually secondary to Medicare. You must have a primary insurer to be assured of coverage, so if this is your situation, it is important to enroll in Medicare when you turn 65. If you become eligible for Medicare because of disability, the only difference is that if the company has 100 or more employees, the group health plan coverage is primary, under 100 employees, it is secondary. Always confirm the type of coverage you have with your employer.