Medicare Basics

Medicare basics come down to four parts, two main coverage paths, and one enrollment deadline that can affect your costs for the rest of your life. Medicare is the federal health insurance program for people age 65 and older, and for some younger people with qualifying disabilities. Getting the Medicare basics straight before you enroll helps you avoid coverage gaps and late penalties you cannot undo later.

This guide covers what Part A, Part B, Part C, and Part D each pay for, how Original Medicare compares to Medicare Advantage, where Medicare Supplement (Medigap) insurance fits, and when you need to sign up.

The four parts of Medicare

Part A is Medicare's hospital insurance. It covers inpatient hospital stays, skilled nursing facility care, hospice, and limited home health services. According to Medicare.gov, most people qualify for premium-free Part A if they or a spouse paid Medicare payroll taxes for at least 10 years.

Part B is Medicare's medical insurance. It covers doctor visits, outpatient care, preventive services, and durable medical equipment. Part B carries a monthly premium set each year by the Centers for Medicare & Medicaid Services (CMS), the federal agency that administers Medicare. See our guide to Part B coverage for what it includes.

Part C, known as Medicare Advantage, is an alternative to Original Medicare sold by private insurers under contract with Medicare. These plans typically bundle Part A, Part B, and often Part D drug coverage into a single plan, and many add extras like dental, vision, and hearing benefits. Our dental, vision, and hearing coverage guide covers how those extras usually work.

Part D is Medicare's standalone prescription drug coverage, sold by private insurers to help pay for medications. You can pair a Part D plan with Original Medicare, or get drug coverage built into a Medicare Advantage plan instead.

Two main paths for coverage

Original Medicare, meaning Part A and Part B together, lets you see any provider that accepts Medicare anywhere in the country. Most people who choose this path add a standalone Part D drug plan and a Medigap supplement to help cover the deductibles and coinsurance Original Medicare leaves behind.

Medicare Advantage, or Part C, replaces how you get your Part A and Part B benefits through a private plan, usually built around a network of providers. Many Medicare Advantage plans include drug coverage and extra benefits, often for a lower monthly premium than Original Medicare plus Medigap.

Neither path is automatically the better choice. Cost, provider access, and drug needs all factor into the decision. Our Compare Medicare Plans guide walks through the trade-offs side by side, and our Medicare Advantage vs. Supplement comparison looks at cost differences in more detail.

Where Medigap fits

Medigap, also called Medicare Supplement insurance, works only alongside Original Medicare. It helps pay the deductibles and coinsurance that Part A and Part B leave behind, which makes your out-of-pocket costs more predictable. You cannot use a Medigap policy with a Medicare Advantage plan.

If controlling unpredictable medical bills matters more to you than a low monthly premium, a Medigap policy paired with Original Medicare is worth comparing against Medicare Advantage. See our guide to Medicare Supplement Plan G for a closer look at one common Medigap option.

When to enroll and how to get started

Most people become eligible for Medicare at 65. According to CMS, the Initial Enrollment Period lasts seven months: it starts three months before your 65th birthday month, includes your birthday month, and ends three months after. Enrolling within this window helps you avoid the Part B and Part D late enrollment penalties, which can raise your premiums for as long as you have Medicare.

If you are still working and covered by a qualifying employer group health plan, you may be able to delay Part B enrollment without a penalty. Our Turning 65 guide covers that timeline step by step. If you think you already missed your window, see how the late enrollment penalty is calculated. Outside your Initial Enrollment Period, the Annual Enrollment Period, which runs October 15 through December 7 each year, is your main chance to switch between Original Medicare and Medicare Advantage or change your drug plan.

For the official rules on eligibility and enrollment timing, see Medicare.gov and the Centers for Medicare & Medicaid Services.