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Accountable care organizations (ACOs) are a significant innovation in health care delivery and payment. ACOs, formed by various health care providers, including acute care hospitals, expanded through commercial health plans and Medicaid agencies even before Medicare ACOs grew under the Affordable Care Act. CMS launched the Medicare Shared Savings Program (MSSP) and Pioneer ACOs in 2012. The MSSP primarily employed a low-risk payment model, rewarding providers with shared savings for meeting spending and quality benchmarks without risking losses. In contrast, the Pioneer ACO program utilized a moderate-risk model, sharing both savings and losses, which saw modest growth in provider participation. Early evaluations indicate some savings and quality improvements in CMS ACOs, but research on Medicaid ACOs’ effects is limited. Commercial ACOs show mixed results in spending and quality of care. The number of ACOs with multiple payer contracts has increased over time, but there is no research distinguishing whether treatment costs and quality of care are affected by ACO maturity (ie, as the number and risk level of contracts and time in accountable care arrangements increase).