As of January 2016, there were 433 Medicare Shared Savings Program (MMSP) Accountable Care Organizations (ACOs) with almost 7.7 million assigned beneficiaries and more than 14,000 participants (a participant may be a group or an individual). Most of these ACOs are one-sided model ACOs that may generate shared savings and do not involve shared losses (Track 1 ACOs).

Importantly, Track 1 ACOs are not considered advanced alternative payment models (APMs) for purposes of MACRA. As a result, a clinician participating in a Track 1 ACO is subject to the merit-based incentive payment system (MIPS) just like a clinician that is not in an ACO (a participant in an advanced APM is not subject to MIPS).