Medicare Shared Savings Program

The Medicare Shared Savings Program (MSSP) is the primary Medicare ACO program. The MSSP was derived from the Physician Group Demonstration Project, which started during the George W. Bush administration, and the MSSP was permanently authorized by the Patient Protection and Affordable Care Act. The MSSP began in 2012 with two participation options, Track 1 and 2. MSSP Track 3 was added in 2016 and the newest addition is Track 1+, which is detailed on this webpage. Track 1 has no downside risk and ACOs can only participate in Track 1 for a limited number of years. Tracks 1+, 2 and 3 all require certain levels of downside risk but have other benefits such as greater amounts of shared savings rewards and waivers from certain Medicare rules.


  • CMS MSSP webpage and federal regulations governing the MSSP
  • NAACOS ACO Comparison Chart Updated for 2018 
  • NAACOS Overview of Research on ACO Performance resource catalogs key ACO studies Updated for 2017
  • NAACOS’s paperDifferences Between Medicare ACO Tracks that May Impact ACO Financial Results
    • NAACOS interactive table for ACOs to input their own projected values to help ACOs illustrate how they may perform under the various tracks. 

News and Advocacy


  • NAACOS and five other provider organizations send a letter to CMS requesting that ACOs meeting cost or quality criteria be able to remain in Track 1

  • NAACOS comments on Extreme and Uncontrollable Circumstances policy for ACOs
    NAACOS letter to CMS identifies PY2016 MSSP calculation error affecting ACOs with previous savings and rebased benchmarks in 2016
  • CMS announces 561 MSSP ACOs in 2018


  • CMS releases interim final rule establishing extreme and uncontrollable circumstances policies for the Shared Savings Program that will apply to ACOs subject to extreme and uncontrollable events such as Hurricanes Harvey, Irma, and Maria, and the California wildfires, effective for performance year 2017
  • New! Highlights of the 2016 Medicare ACO performance results
  • CMS releases performance year 2016 MSSP results, available in this Public Use File and CMS Presentation
  • NAACOS letter expresses concerns about delayed PY 2016 results, urges CMS action
  • NAACOS letter to CMS Administrator Seema Verma on ACO improvements
  • NAACOS policy paper with detailed recommendations for improving the Medicare ACO program
  • CMS Press Release on Announcement of 2017 ACO Class
  • NAACOS resource on key policy issues for ACOs in 2017
  • Talking Points on the Important Role of ACOs in Health Care Delivery Reform 
  • Performance Year 2015 Results