Objective To identify factors that promote the effective performance of accountable care organizations (ACOs) in the Medicare Shared Savings Program. Data Sources/Study Setting Data come from a convenience sample of 16 Medicare Shared Savings ACOs that were organized around large physician groups. We use claims data from the Center for Medicaid and Medicare Services and data from 60 interviews at three high‐performing and three low‐performing ACOs. Study Design Explanatory sequential design, using qualitative data to account for patterns observed in quantitative assessment of ACO performance. Data Collection Methods A total of 16 ACOs were first rank‐ordered on measures of cost and quality of care; we then selected three high and three low performers for site visits; interview data were content‐analyzed. Principal Findings Results identify several factors that distinguish high‐ from low‐performing ACOs: (1) collaboration with hospitals; (2) effective physician group practice prior to ACO engagement; (3) trusted, long‐standing physician leaders focused on improving performance; (4) sophisticated use of information systems; (5) effective feedback to physicians; and (6) embedded care coordinators. Conclusions Shorter interventions can improve ACO performance—use of embedded care coordinators and local, regional health information systems; timely feedback of performance data. However, longer term interventions are needed to promote physician–hospital collaboration and skills of physician leaders. CMS and other stakeholders need realistic timelines for ACO performance.