Back in August of 2018, as part of the Medicare Shared Savings Program (Shared Savings Program), CMS proposed some sweeping changes for Accountable Care Organizations (ACOs). There has been some controversy over these changes which require ACOs to move to two-sided models. In this Final Rule which was scheduled to be published in the Federal Register on December 31, 2018, they address the comments and concerns submitted during the comment period.
For those that participate in ACOs, be aware that even though the majority of the provisions are effective 60 days after the rule is published, there are some which have different effective dates. Those provisions can be found in Table 1 of the CMS Final Rule.
The referenced article includes a good overview.