Measure sets developed by the ACC/AHA are intended to provide practitioners and institutions that deliver cardiovascular services with tools to measure the quality of care provided and identify opportunities for improvement. Writing committees are instructed to consider the meth- odology of performance measure development1 and to ensure that the measures developed are aligned with ACC/AHA clinical practice guidelines. The writing committees also are charged with constructing measures that maximally cap- ture important aspects of care quality, including timeliness, safety, effectiveness, efficiency, equity, and patient-centered- ness, while minimizing, when possible, the reporting burden imposed on hospitals, practices, and/or practitioners. Potential challenges from measure implementation may lead to unintended consequences. The manner in which chal- lenges are addressed is dependent on several factors, includ- ing the measure design, data collection method, performance attribution, baseline performance rates, reporting methods, and incentives linked to these reports. The ACC/AHA Task Force on Performance Measures (Task Force) distinguishes quality measures from performance measures. Quality measures are those metrics that may be use- ful for local quality improvement but are not yet appropriate for public reporting or pay for performance programs (uses of performance measures). New measures are initially evalu- ated for potential inclusion as performance measures. In some cases, a measure is insufficiently supported by the guidelines. In other instances, when the guidelines support a measure, the writing committee may feel it is necessary to have the measure tested to identify the consequences of measure implementation. Quality measures may then be promoted to the status of per- formance measures as supporting evidence becomes available. 