CMS stands for the Centers for Medicare & Medicaid Services, the federal agency that runs Medicare along with several other major health coverage programs. It provides health coverage to more than 160 million people through Medicare, Medicaid, the Children’s Health Insurance Program, and the Health Insurance Marketplace. If you’ve ever used any of these programs or visited Medicare.gov, you’ve interacted with CMS.
What CMS Actually Does
CMS is part of the U.S. Department of Health and Human Services. Its core job is administering the country’s largest health insurance programs, but it also sets rules that ripple across the entire healthcare system. It establishes payment rates for hospitals and doctors, regulates private insurers that participate in Medicare, and works to improve quality and equity in healthcare delivery. When you hear about changes to Medicare coverage, new drug pricing rules, or updates to marketplace insurance, those decisions typically come from CMS.
How Medicare Works
Medicare is the federal health insurance program primarily for people 65 and older. You also qualify before 65 if you’ve received Social Security disability benefits for 24 months, have ALS (Lou Gehrig’s disease), or have end-stage renal disease requiring dialysis or a kidney transplant. For ALS, coverage begins as soon as disability benefits start, with no waiting period.
The program is divided into four parts, each covering different types of care.
Part A: Hospital Insurance
Part A covers inpatient hospital stays, skilled nursing facility care, hospice care, and some home health care. Most people don’t pay a premium for Part A if they or a spouse paid Medicare taxes while working. You’re enrolled automatically when you turn 65 if you’re already receiving Social Security benefits.
Part B: Medical Insurance
Part B covers doctor visits, outpatient care, preventive services like screenings and vaccines, durable medical equipment (wheelchairs, walkers, hospital beds), and home health care. Unlike Part A, Part B comes with a monthly premium. The standard premium is $185 in 2025 and rises to $202.90 in 2026. There’s also an annual deductible: $257 in 2025, increasing to $283 in 2026. Higher-income enrollees pay more.
Part C: Medicare Advantage
Medicare Advantage plans are an alternative way to get your Medicare benefits. Instead of using Original Medicare (Parts A and B) directly, you enroll in a plan run by a private insurance company that contracts with CMS. These plans bundle Part A, Part B, and usually Part D drug coverage into a single plan. Many also include extras like dental, vision, or hearing coverage that Original Medicare doesn’t offer. The tradeoff is that Advantage plans typically use provider networks, meaning you may need to see specific doctors or get referrals.
Part D: Prescription Drug Coverage
Part D helps cover the cost of prescription drugs. You can get it two ways: as a standalone drug plan that pairs with Original Medicare, or bundled into a Medicare Advantage plan. Private insurers run these plans under rules set by CMS, and each plan varies in cost and which drugs it covers. Starting in 2025, insulin costs under Part D are capped at $35 per month, a change that carries forward into 2026 and beyond. CMS also now negotiates prices directly with manufacturers on certain high-cost drugs through the Medicare Drug Price Negotiation Program.
When and How to Enroll
Your Initial Enrollment Period is a seven-month window that starts three months before the month you turn 65 and ends three months after. If you’re already collecting Social Security, you’ll be enrolled in Parts A and B automatically at 65. Otherwise, you need to sign up yourself during this window.
Missing your enrollment window has real consequences. You may have to wait for a later enrollment period, and you’ll likely face a late enrollment penalty added to your monthly Part B premium for as long as you have coverage. The penalty increases the longer you delay. If you’re still working at 65 and have employer coverage, special rules let you delay enrollment without penalty, but the timing matters.
Filling Gaps With Medigap
Original Medicare doesn’t cover everything. You’re still responsible for deductibles, copayments, and coinsurance, which can add up quickly during a hospital stay or ongoing treatment. Medigap policies, also called Medicare Supplement Insurance, are sold by private insurers specifically to cover these out-of-pocket costs. When you have a Medigap policy, Medicare pays its share first, then Medigap picks up some or all of the remainder.
To buy a Medigap policy, you need to be enrolled in both Part A and Part B. You’ll pay the Part B premium plus a separate monthly premium to the Medigap insurer. One important detail: Medigap plans work only with Original Medicare. If you choose a Medicare Advantage plan instead, you can’t use a Medigap policy alongside it.
Who Qualifies Before Age 65
While most people associate Medicare with turning 65, several groups qualify earlier. People receiving Social Security disability benefits get Medicare automatically after 24 months of benefits. Those diagnosed with ALS skip the waiting period entirely and receive Medicare as soon as disability benefits begin.
End-stage renal disease is another path to early eligibility regardless of age. You qualify if your kidneys no longer function and you need regular dialysis or have had a kidney transplant, provided you or a spouse have a sufficient work history under Social Security or are already receiving Social Security or Railroad Retirement benefits. Dependent children of qualifying workers are also covered.
How CMS Shapes What Medicare Covers
CMS doesn’t just process claims. It actively shapes the cost and scope of Medicare through policy changes each year. Recent changes from the Inflation Reduction Act, for example, introduced the $35 monthly cap on insulin and created a framework for CMS to negotiate drug prices directly with pharmaceutical companies. For 2025, Medicare Part D also introduced a $2,000 annual cap on out-of-pocket drug spending, a significant shift for people taking expensive medications.
Each year CMS announces updated premiums, deductibles, and coverage rules that affect every Medicare enrollee. These updates typically take effect in January, and CMS publishes them months in advance so people can make informed choices during the annual Open Enrollment Period, which runs from October 15 through December 7.