Improving care coordination has emerged as a key strategy of many payers and policymakers for enhancing the quality and lowering the costs of health care in the United States. Accountable care organizations and bundled payments aim to bridge the provider-based silos that fragment care. Patient-centered medical homes (PCMHs) seek to coordinate the care of patients with chronic disease. And recent changes to Medicare physician payments provide explicit incentives to enhance transitional care, such as the transition from hospital to outpatient care. To determine if care coordination does indeed have positive effects on quality or cost, the first step is to decide how it can be measured.