The Centers for Medicare and Medicaid Services (CMS) recently finalized changes to the Medicare Shared Savings Program (MSSP), its primary accountable care organization (ACO) program and most significant approach to constraining ballooning program costs while improving quality. These changes will require ACOs to transition more quickly to two-sided risk. The program has continued to grow and demonstrated some savings, as detailed in recent posts (Part 1 and Part 2), with 10.5 million beneficiaries and 561 MSSP ACOs in 2018.