Enthusiasm for accountable care organizations (ACOs) is high among many policymakers and health industry leaders seeking ways to provide better care at lower cost to more patients. As ACOs take hold, it will be critically important to track their performance over time and ensure that patients from all backgrounds can benefit from these new models. In a new Commonwealth Fund–supported study in Health Affairs, researchers report baseline information collected on patient populations, quality, participating hospitals, and costs for organizations participating in Medicare's ACO programs as of fall 2012.