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In 2012, the Centers for Medicare and Medicaid Services (CMS) launched new voluntary payment models for Accountable Care Organizations (ACOs) with incentives to control spending. The number of Medicare ACOs has grown steadily to more than 600 organizations responsible for managing care for nearly 12 million Medicare beneficiaries in 2018. Similar programs have been established in the commercial, Medicare Advantage, and Medicaid markets. However, the majority of Medicare ACOs participate in Track 1 of the Medicare Shared Savings Program (MSSP) under a “one-sided” arrangement – also known as “upside-only” – where ACOs earn shared savings payments if their annual spending is below a target budget known as a benchmark, but are not required to repay Medicare if their spending exceeds the benchmark. In 2018, about 25 percent of Medicare ACOs were in MSSP Tracks 1+, 2, and 3 or in the Next Generation ACO model, which are “two-sided” or “shared” risk arrangements. ACOs participating in these models can generally earn larger shared savings payments if they are successful, but they also face “downside” risk because they are responsible for repaying a portion of any losses to the government.