HealthCare Partners, a medical group and independent practice association in Los Angeles, is one of the provider groups taking part in the Brookings–Dartmouth ACO Pilot Program to form accountable care organizations, which assume responsibility for improving patient care and lowering total costs and, in turn, share in the savings achieved. This case study explores: the characteristics of HealthCare Partners and its partner organizations, including the insurer Anthem, that are contributing to the development of the ACO; the rationale behind the decision to develop an ACO; the steps taken to implement the model; milestones achieved; and lessons learned. The keys to HealthCare Partners' success thus far include a strong primary care base, an emphasis on prevention and promotion, an integrated health information technology infrastructure, chronic care management and care coordination efforts, performance measurement and reporting, and experience with taking on risk.