Medicare Shared Savings Program (MSSP)

Key facts

Legal basis

Section 1899 of the Social Security Act; regulations at 42 CFR Part 425

Started

First ACO agreements began in 2012

Tracks

BASIC (Levels A through E) and ENHANCED

Agreement period

Generally five years

Minimum size

At least 5,000 assigned beneficiaries

What is the Medicare Shared Savings Program?

The Shared Savings Program was created by the Affordable Care Act as a permanent part of Medicare, unlike the time-limited models tested by the CMS Innovation Center. It lets groups of providers form an accountable care organization (ACO), take responsibility for a population of Original Medicare beneficiaries, and share in savings measured against a financial benchmark.

Participation is voluntary for providers. Beneficiaries do not enroll: they are assigned to an ACO based on their use of primary care services or through voluntary alignment, and they remain free to see any provider that accepts Medicare.

How the Shared Savings Program is structured

ACOs choose a track that sets how much financial risk they take:

• BASIC track: a glide path from upside-only levels (A and B) to levels with increasing downside risk (C, D and E), with lower maximum sharing rates than ENHANCED.
• ENHANCED track: the highest level of two-sided risk and the highest share of savings.
• Benchmarks: based on historical Parts A and B spending for assigned beneficiaries, adjusted for risk and regional trends.
• Quality: ACOs report quality measures, and performance affects how much of any savings they keep.

Why the Shared Savings Program matters

The MSSP is the largest accountable care program in Original Medicare, which makes its public files a practical map of accountable care. CMS publishes ACO names, participant organizations and financial and quality results, so analysts can link practices and hospitals to their ACOs and see which organizations earned savings.

An ACO in a two-sided risk track has a direct financial reason to manage hospital use and high-cost drug and device spending, which matters for commercial teams deciding whom to engage. CMS revises program rules through annual Physician Fee Schedule rulemaking, so benchmark methods and quality reporting change from year to year.

Shared Savings Program vs ACO REACH

The Shared Savings Program is set in statute and has no end date. ACO REACH is a time-limited CMS Innovation Center model with higher risk options, including capitation, scheduled to end after performance year 2026. CMS has announced the LEAD Model as the Innovation Center successor to ACO REACH starting in 2027, while the Shared Savings Program continues.