Turns out, there are a number of big health expenses that Original Medicare, the federal health insurance program for Americans over 65, doesn’t cover. But—and it’s a big but—for most of those there’s often an exception to the rule.

It’s also worth noting that Medicare Advantage plans—the private-insurance alternative to Original Medicare—sometimes offer coverage that Original Medicare doesn’t. But, as you’ll see below, that coverage may be skimpier than you think because Medicare Advantage benefits aren’t standardized.

“Medicare Advantage plans are required to cover everything Original Medicare covers and nothing more. A supplemental benefit could be robust, or it could be not,” says Casey Schwarz, senior counsel for education and federal policy at the Medicare Rights Center. “We hear from a lot of people who are surprised by what their Medicare Advantage plan doesn’t cover.”

The government’s free annual handbook, Medicare & You, and the Medicare website, provide specifics about what Original Medicare does and doesn’t cover. The Medicare.gov site has a handy tool called “Is my test, item, or service covered?” where you type in a word or phrase to get the answer. State Health Insurance Assistance Programs, or SHIPs, also provide free information about Medicare rules—you can find the program in your state with a quick Google search.

Here are big coverage gaps to look out for, what to know about them, and how to appeal a Medicare claim denial if you believe your care is covered:

The big 3: Vision, dental, and hearing

Original Medicare doesn’t cover routine medical expenses for your eyes, your mouth, or your ears. You generally can’t get reimbursed for annual visits to the dentist or optometrist. And forget about coverage for new eyeglasses or hearing aids. But some types of vision, dental, and hearing expenses are covered.

For vision care, “it’s really pretty pathetic what Medicare covers,” says Diane Omdahl, founder and president of 65 Incorporated, a Medicare advisory group.

But Original Medicare will cover what are considered “medically necessary services” such as treating an eye injury; cataract surgery; corrective lenses for eyeglasses or contacts after cataract surgery; and annual glaucoma tests if you’re at high risk for the eye disease.

Medicare Advantage plans may say they cover routine eye exams and eyeglasses. “But their allowance for glasses might be $150 to $300,” says Omdahl. “Do you want the right side or the left side of your glasses?” she jokes.

Original Medicare won’t pay for teeth cleanings, dentures, root canals, or annual X-rays. But it does cover X-rays and dental surgery after an accident, treatment for mouth cancers, and dental exams needed before kidney or heart surgery.

“If you are in a car accident and have to have your teeth taken out, Original Medicare will cover that,” says Omdahl, because it’s medically necessary. “But they won’t cover putting them back in,” since that isn’t, she adds.

Medicare Advantage plans sometimes provide dental insurance, but Schwarz says often their dental benefit “is actually very limited or amounts to a discount on dental service but not robust coverage.”

Some Medicare Advantage plans cover just two exams a year and maybe one cleaning, notes Omdahl. “Some plans will cover comprehensive dental care, but no implants. And they set an annual dollar limit of $2,000 or so out-of-pocket.”

Not all dentists are enrolled in Medicare, though, so Schwarz advises seeing whether your dentist is and can then submit claims for you.

Although Original Medicare usually won’t cover hearing exams, it might provide reimbursement if your hearing is creating a medical problem. “If you have a balance problem, they’ll cover hearing, but they won’t cover hearing if you just can’t hear,” says Omdahl.

Foot care

Here, you need to tread carefully. Original Medicare won’t cover routine foot exams, corns, calluses, or flat feet.

But it does cover therapeutic shoes for diabetes treatment and medically necessary treatment for foot injuries or diseases, such as hammer toe, heel spurs, and foot warts. It also covers yearly exams or treatment if you have diabetes-related lower leg nerve damage.

Coverage for the cost of canes and walkers? That depends.

“If you have a diagnosis and a prescription and can get the equipment from a Medicare-approved supplier, Medicare will cover canes and walkers,” says Schwarz. “But you have to have an ambulatory need within your home.”

Plastic surgery

Forget trying to get Original Medicare to pay for a tummy tuck, breast enhancement, or a nose job. Those types of cosmetic procedures aren’t covered.

But, if you need plastic surgery because you were burned or in an accident or had complications from surgery, Original Medicare covers that. “If you can’t breathe and need rhinoplasty on your nose, it’s covered. But if it’s just to make you prettier, then it’s not,” says Omdahl.

Acupuncture

Not to put too fine a point on it, but whether Original Medicare will cover your acupuncture depends on why you’re getting it.

Normally, acupuncture isn’t covered, but there’s one exception: if your doctor prescribes it for your chronic low back pain. But then, you’ll only be reimbursed for up to 12 sessions over 90 days (eight more if you’re showing improvement), with a maximum of 20 treatments over a 12-month period.

Chiropractic care

In most cases, Original Medicare won’t cover going to the chiropractor. But there’s an exception, of course: treatment for some spinal diagnoses.

Medical care overseas

Generally, Original Medicare won’t kick in if you have health expenses while traveling outside the United States, though some Medigap and Medicare Advantage policies might.

But Original Medicare may pay for services on a cruise ship if you’re in a U.S. port or within six hours of one when the ship arrives or departs.

Long-term care and home care

By and large, Original Medicare doesn’t cover most long-term care costs and only covers some home care expenses when they are for medical reasons. In other words, it won’t pay for what are called “activities of daily living,” such as help with dressing, bathing, or meals.

“I think that can be surprising, especially to family members taking on the management of a loved one’s care for the first time, such as for dementia," says Schwarz.

Medicare considers an assisted living community the patient’s residence. “The criteria for getting Original Medicare coverage in an assisted living facility is the same as if you are in your home,” says Omdahl.

When Omdahl’s father was in a skilled nursing facility and had kidney cancer, Medicare paid for the care for drainage of nephrectomy tubes going into his kidney. “But as soon as the tubes were implanted and there was no longer a need for a nurse to take care of him, it all went into private pay,” she notes.

Original Medicare will, however, pay for a home health aide if you’re homebound and, say, recovering from a stroke and getting physical therapy or need some other type of skilled medical care.

Some hospital costs

Although Medicare Part A covers hospital costs, there are serious limits.

For instance, Original Medicare typically won’t pay for a private room in a hospital, only a semiprivate room (unless there are no semiprivate rooms or a private room is deemed medically necessary).

It also won’t pay for private-duty nursing if you feel you need extra nursing coverage or for a TV or phone in your hospital room if there’s a separate charge for those.

Appealing a Medicare claim denial

If Original Medicare denies a claim that you believe is legitimate, you can appeal by submitting a Redetermination Request Form. And Omdahl and Schwarz think you should, if you have a good case.

“If Medicare says, ‘This was not medically necessary for you; we don’t think you had the diagnosis that requires this treatment’ and you can show you did, your chance of a successful appeal is high,” says Schwarz.

Omdahl, who previously worked in home care, says: “In my experience, the appeal system is very beneficiary friendly. I tell people: Just fill out the form the best you can.”

This story was originally featured on Fortune.com

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