The desire to improve the quality of health care has prompted the increased use of performance measures. These measures examine the outcomes of interventions for evidence of improved health and are used to hold providers accountable for the quality of health care. Yet tension exists regarding the capabilities of the current science of quality measurement. Physicians and researchers often question the validity of these data-for example, the use of hospital claims to measure quality-even as policy makers, payers, and the public seek evidence of improved performance. This article discusses the current impasse in the field of quality measurement and what is needed to overcome this deadlock.