Accountable care organizations (ACOs) were originally developed as a Medicare construct as part of the Patient Protection and Affordable Care Act (PPACA) to improve care coordination and lower the cost of care. At this time, four Medicare ACO models have been developed: the Medicare Shared Savings Program, the Pioneer ACO model, the Next Generation ACO model, and the Advanced Payment/Investment ACO model. This report provides a longitudinal look at Medicare ACOs. First, a look at the evolution of Medicare ACOs over time and the six models for financing. Second, a look at the changes in Medicare ACO enrollment over . . .