Some people may get Medicare Part A "premium-free," but most people have to pay a monthly premium for Medicare Part B. Because Medicare Part B comes with a monthly premium, some people may choose not to sign up during their initial enrollment period if they are currently covered under an employer group plan (either their own or through their spouse's employer).

If you are still working, you should check with your health benefits administrator to see how your insurance would work with Medicare. If you delay enrollment in Medicare Part B because you already have current employer health coverage, you can sign up later during a Special Enrollment Period without paying a late penalty. You can enroll in Medicare Part B at any time that you are still covered by a group plan based on current employment. After your employer health coverage ends or your employment ends (whichever comes first), you have an eight-month special enrollment period to sign up for Part B without a late penalty.

Keep in mind that retiree coverage and COBRA are not considered health coverage based on current employment and would not qualify you for a special enrollment period. If you have COBRA after your employer coverage ends, you should not wait until your COBRA coverage ends to sign up for Medicare Part B. Your eight-month Part B special enrollment period begins immediately after your current employment or group plan ends (whichever comes first). This is regardless of whether you get COBRA.

Medicare doesn't cover everything. If you need certain services that Medicare doesn't cover, you'll have to pay for them yourself unless you have other insurance or you're in a Medicare health plan that covers these services.

Even if Medicare covers a service or item, you generally have to pay your deductible, coinsurance, and copayments.

Some of the items and services that Medicare doesn't cover include:

• Long-term care (also called custodial care)
• Most dental care
• Eye examinations related to prescribing glasses
• Dentures
• Cosmetic surgery  
• Acupuncture  
• Hearing aids and exams for fitting them
• Routine foot care

Most plans with Medicare prescription drug coverage (Part D) have a coverage gap (called a "donut hole"). This means that after you and your drug plan have spent a certain amount of money for covered drugs, you have to pay all costs out-of-pocket for your prescriptions up to a yearly limit. Once you have spent up to the yearly limit, your coverage gap ends and your drug plan helps pay for covered drugs again.

If you already get Social Security benefits, you will be automatically enrolled in Medicare Hospital Insurance (Part A) and Medical Insurance (Part B). You will be mailed all the information you need a few months before you become eligible.

If you don’t get Social Security benefits and are not ready to apply for them yet, you should sign up for Medicare three months before your 65th birthday.