Those 65 and older are eligible for Medicare Plans: Medicare Part A, Medicare Part B and Medicare Part D. Although these "parts" of Medicare will cover a portion of your medical bills, they do not cover all of them. The items that Medicare doesn't cover are called the "gaps" in Medicare. This is why Medicare sometimes calls insurance supplements a "Medigap" policy. These gaps include deductibles, co-insurance, co-pays and excess charges. These can add up very quickly, resulting in costly medical bills. Medicare Supplement Insurance provides added peace of mind. It pays most (or all) of the gaps in Medicare depending on the Medicare supplement plan selected. Beneficiaries can enroll in Medigap Insurance at various times, including specific times where they are in what is called "guaranteed issue period", which allows them to enroll with no medical pre-screening required. They are essentially guaranteed coverage and a policy, and that policy is guaranteed renewable - regardless of one's own health claims or health status.