Predictably, the work has proved challenging for both CMS and ACOs, even those with extensive experience in population care management. While the withdrawal of any participant is concerning and needs to be better understood, individual organizations must make decisions most appropriate for their particular market and strategic circumstances, and the majority of those leaving the model have or plan to transition to the other Medicare ACO program (the Medicare Shared Savings Program). The Pioneer model continues to mature, fueled by rapid cycles of measurement, reporting, learning, and refinement made possible by the close collaboration CMS has formed with participating ACOs. Performance is on an upward trajectory in a manner aligned with original expectations, as ACOs become more strategic and effective in implementing care strategies and as CMS becomes more effective at facilitating their work. Looking forward, CMS will apply lessons learned about the clinical and technical sophistication, and the persistent and sustained approach that ACOs need to maintain care transformation, to the development of new models such as those that engage ACOs in global payment arrangements that offer ACOs more tools for directly engaging patients in care improvement. CMS will evaluate whether these Pioneer ACO results warrant expansion nationally. Early success in the Pioneer model suggests that in the long term, accountable care will offer patients the improved outcomes they deserve and ACOs the sustainable business model they need to stay focused on delivering high-value care.