This case study series describes how three diverse organizations are developing accountable care systems to improve the quality and reduce the costs of care, and ultimately improve the health of populations of patients insured by Medicare, Medicaid, and commercial health plans. They employ a constellation of strategies to identify and address unmet medical needs, improve care transitions, and reduce inefficiencies and unnecessary variation in care. Care managers, outreach workers, or virtual care teams help improve outcomes for patients with complex needs that are costly to treat. Data integration and analytics are key to their efforts, although the sophistication of these capabilities varies. Two study sites have established a record of savings, while the third is still proving the potential of its approach. Their progress to date suggests that payment reforms can foster the will and accountability necessary to transform care.