Who Runs Medicare and How the System Works

Medicare is run by the federal government, but three different agencies handle different parts

Medicare is a federal health insurance program, which means it is funded and overseen by the U.S. government, not by private insurance companies. However, no single agency runs all of it. The Centers for Medicare & Medicaid Services (CMS), a division of the Department of Health and Human Services, sets the rules and manages the program overall. But the day-to-day work of paying doctors, processing claims, and handling customer questions is split among regional contractors and private insurance companies that CMS hires to do the work.

Understanding who does what matters because it changes who you contact when something goes wrong. If your claim was denied, you may need to contact the contractor who processed it, not CMS itself. If you have a question about your coverage, the answer depends on which part of Medicare you are in — Original Medicare, Medicare Advantage, or a prescription drug plan — because each one is administered differently.

Key Takeaways

The Centers for Medicare & Medicaid Services (CMS) is the federal agency that oversees Medicare, but it does not process claims or handle most customer service directly.
Original Medicare (Parts A and B) is administered by regional contractors called Medicare Administrative Contractors (MACs) who process claims and answer coverage questions in your area.
Medicare Advantage (Part C) plans are run by private insurance companies that CMS contracts with, and you contact your plan directly for claims and coverage decisions.
Prescription drug plans (Part D) are also run by private insurance companies, and each plan has its own customer service line and coverage rules.
The Social Security Administration handles Medicare enrollment and premium payments, even though CMS runs the program itself.

The Centers for Medicare & Medicaid Services (CMS) sets policy but does not process your claims

CMS is the federal agency inside the Department of Health and Human Services that writes the rules for Medicare, decides what services are covered, sets payment rates for doctors and hospitals, and monitors the contractors who do the actual work. CMS has regional offices across the country, but they do not handle individual claims or customer service calls.

If you call CMS directly, you will reach a general information line that can answer broad questions about how Medicare works, but they cannot look up your specific claim or tell you why a particular service was denied. For those questions, you need to contact the contractor or insurance company that actually processed your claim.

Medicare Administrative Contractors (MACs) handle Original Medicare claims and coverage decisions

If you are in Original Medicare (Parts A and B), your claims are processed by a Medicare Administrative Contractor (MAC) assigned to your state or region. MACs are private companies that CMS contracts with to handle the administrative work. There are 12 regional MACs across the country, and each one covers a specific geographic area. Your MAC is responsible for processing claims from doctors and hospitals, paying them, and answering questions about whether a service is covered.

When a doctor submits a claim for a service you received, it goes to your regional MAC, not to CMS. The MAC decides whether to pay it based on Medicare rules. If the claim is denied, the MAC sends you a notice explaining why. If you disagree with the decision, you file an appeal with the same MAC. You can find your regional MAC on the Medicare website by entering your state, or you can call 1-800-MEDICARE and ask.

MACs also publish local coverage decisions (LCDs), which are rules about what services Medicare will pay for in your specific region. These can differ from national rules, so a service might be covered in one state but not another. Your MAC's website lists these decisions, and you can call them to ask whether a specific service is covered before you have it done.

Private insurance companies run Medicare Advantage and prescription drug plans

If you choose Medicare Advantage (Part C), you are not dealing with CMS or a MAC directly. Instead, you are enrolled in a plan run by a private insurance company — companies like UnitedHealthcare, Humana, Aetna, or Anthem. CMS contracts with these companies to offer Medicare coverage, but the insurance company is your primary contact. You call your plan's customer service line to ask about coverage, file claims, or appeal a denial.

The same is true for prescription drug plans (Part D). These are run by private insurance companies, and each plan has its own rules about which drugs are covered and what you pay. You contact your Part D plan directly, not CMS or a MAC, if you have questions about your medications or need to appeal a coverage decision.

Because Medicare Advantage and Part D plans are run by private companies, the experience can vary significantly from plan to plan. One plan might cover a drug that another plan does not. One plan's customer service might be faster than another's. This is why comparing plans during the annual enrollment period matters — you are comparing not just the coverage, but also the company that will be administering your benefits.

The Social Security Administration handles enrollment and premium payments

When you first become may be able to access for Medicare, you sign up through the Social Security Administration (SSA), not through CMS. Social Security handles enrollment, collects your premiums, and maintains your enrollment record. If you need to change your enrollment — for example, switching from Original Medicare to Medicare Advantage — you do that through Social Security or during the annual enrollment period managed by CMS.

Your Medicare premiums are deducted from your Social Security check (if you receive one) or billed to you directly. If you have a question about your premium amount or payment, Social Security is the agency to contact. However, if you have a question about what your premium covers or why a service is not covered, you contact CMS, your MAC, or your insurance plan depending on which part of Medicare you are in.

The 1-800-MEDICARE line connects you to the right place

If you are unsure who to contact, call 1-800-MEDICARE (1-800-633-4227). This is a federal customer service line run by CMS. The representatives there cannot process claims or make coverage decisions, but they can answer general questions about how Medicare works, tell you which MAC covers your area, help you find your Medicare Advantage or Part D plan's customer service number, and direct you to the right place for your specific question.

The line is open 24 hours a day, seven days a week. If you are deaf or hard of hearing, you can use the TTY number 1-877-486-2048. Having your Medicare card handy when you call makes it faster for the representative to help you.

State insurance counseling programs offer free help navigating the system

Every state has a State Health Insurance information Program (SHIP) that offers free counseling to Medicare beneficiaries. SHIP counselors can explain who administers different parts of Medicare, help you understand a denial letter, walk you through the appeal process, and answer questions about coverage. They are funded by the federal government but run by each state, so they know the local contractors and insurance companies in your area.

You can find your state's SHIP by calling 1-800-MEDICARE or by visiting the Eldercare Locator at 1-800-677-1116. SHIP counselors do not work for CMS, Medicare, or any insurance company, so they have no financial interest in which plan you choose. Many offer in-person appointments as well as phone counseling.

Frequently Asked Questions

Can I call CMS directly to ask about my claim?

CMS does not handle individual claims. Call 1-800-MEDICARE for general information, but to ask about a specific claim, you need to contact your regional Medicare Administrative Contractor (if you are in Original Medicare) or your insurance plan (if you are in Medicare Advantage or Part D).

How do I find out which MAC covers my area?

Call 1-800-MEDICARE and tell them your state, or visit the Medicare website and enter your ZIP code. The MAC's contact information and website will appear. You can also find it by searching "[your state] Medicare Administrative Contractor."

If I am in Medicare Advantage, do I still deal with CMS?

CMS oversees Medicare Advantage plans, but you do not contact CMS directly. You contact your insurance plan for claims, coverage decisions, and customer service. CMS sets the rules the plan must follow, but the plan is your day-to-day contact.

Who do I contact if I disagree with a coverage denial?

Contact the organization that denied the claim: your MAC (if Original Medicare), your insurance plan (if Medicare Advantage or Part D), or your prescription drug plan (if Part D). They will explain the appeal process. You can also ask a SHIP counselor for help with the appeal.

Does Medicare charge a fee to contact customer service?

No. Calling 1-800-MEDICARE is free. Calling your MAC or insurance plan is free. SHIP counseling is free. Be cautious of third-party websites or services that charge fees to help you with Medicare questions — you do not need to pay for this information.

This guide is for general information only and is not medical, financial, legal, or other professional advice. For decisions specific to your situation, consult a qualified professional. See our Editorial Policy.