What this is
Medicare is the federal health program for people 65 and older, some younger people with disabilities, and people with end-stage renal disease. It is not one insurance card—it is several parts that patients combine in different ways.
Advocates do not need to memorize every rule on day one. You need to know which part pays for the care in dispute, who issued the denial, and which appeal path applies.
Parts A, B, C, and D
Part A (hospital insurance) covers inpatient hospital stays, skilled nursing facility care after a qualifying hospital stay, hospice, and some home health.
Part B (medical insurance) covers doctor visits, outpatient care, preventive services, durable medical equipment, and many tests and therapies.
Part C (Medicare Advantage) is an alternative way to receive Medicare benefits through a private plan that contracts with the federal government. It replaces Original Medicare for covered benefits and often includes Part D. See Medicare Advantage.
Part D is prescription drug coverage—either a stand-alone drug plan with Original Medicare or bundled in many Advantage plans. See Medicare Part D.
Medigap (Medicare Supplement) is separate private insurance that helps pay cost-sharing under Original Medicare—it is not Part C. See Medigap / Medicare Supplement when it applies.
How people enroll
Many people are automatically enrolled in Parts A and B when they turn 65 and receive Social Security. Others must sign up during Initial Enrollment Period windows. Missing enrollment can mean late penalties for Part B or Part D.
Each year, people can change Advantage or Part D plans during Annual Enrollment (October 15–December 7) and related open enrollment periods. Keep enrollment confirmations—they explain which organization is responsible for denials.
Who runs Medicare
The Centers for Medicare & Medicaid Services (CMS), part of the U.S. Department of Health and Human Services, administers Medicare. Medicare contractors process claims for Original Medicare. Medicare Advantage plans and Part D plans are private companies regulated by CMS.
State insurance departments generally do not decide Medicare benefit appeals the way they do for commercial insurance. For regulator complaints about Medicare, see CMS: Medicare on the Insurance topic.
Common confusion for advocates
- The patient says "I have Medicare" but the card is a private plan—that is often Advantage, not Original Medicare.
- A drug denial may be Part D even when medical care is covered under Part B or Advantage.
- Patients with both Medicare and Medicaid (dual-eligible) have special billing and plan rules. See Dual-eligible patients.
- Employer retiree coverage or COBRA can sit on top of Medicare—ask what pays first.
Where to go next
For private insurance models (employer, Marketplace), see How coverage works by plan type on the Insurance topic.
Bottom line
Medicare is federal, multi-part coverage. Identify whether the patient uses Original Medicare, Advantage, and/or Part D before you choose forms, deadlines, or regulators. The guides above walk through each model and the shared appeal ladder.